Fitting a new radio or head unit

Fitting a modern head unit in an E30 is a one-evening job once you know what each factory wire does. This guide takes the connections from the factory wiring pages, so you can wire a new unit without guessing, and fix it when something does not work.

Before you start: disconnect the battery. The radio plug carries live battery voltage even with the key out, and a slipped probe or screwdriver can blow fuses or damage the unit.

Which system do you have?

There are two layouts in the factory manual. The wires at the radio are the same, but the speaker side differs.

System	How to tell	Wiring page
Basic radio	Radio drives the speakers through a fader control. No amplifier in the boot.	6500-2 Radio / Power Antenna (325e)
Sound system with amplifier	A separate amplifier module in the car. The radio sends a low-level signal to it, and the amplifier drives the speakers.	6500-0 Radio/Antenna with Sound System (radio) and 6500-1 Radio/Antenna with Sound System

Both can have a power antenna. If you are unsure which you have, trace the wires behind the radio or look for an amplifier module in the car. Its position varies.

Removing the old radio

With the battery disconnected, release the radio. Factory radios are held by special pentagon-head bolts. Owners report that a small hex key works on them, and that the bolts only need backing out far enough to free the unit rather than removing them completely.
Slide the radio out and unplug the connectors at the back. The antenna lead pulls straight out. Do not cut the factory plug off. A plug-in adapter harness lets you keep the car's wiring original and reversible.
If the radio has a 7 A fuse on its back panel (the factory diagram shows one), note it. A new unit will have its own fuse.

Power, ground and lights

These are the wires at the radio, from the factory diagrams. Colours are as drawn on the diagram, so confirm each one with a meter before you connect it. The last column is the standard colour on an aftermarket head unit's harness.

Function	Factory wire	Fuse	Aftermarket wire
Constant 12 V (memory and clock)	Red, 1.0 mm² feed, through accessory connector C302 terminal V and radio connector C215 pin 2 (radio terminal 30)	Fuse 21 (7.5 A) and the radio's own 7 A fuse	Yellow
Switched 12 V (key in ACC, RUN or START)	Violet, 0.75 mm², through C302 terminal X to radio connector C216 pin 2	Fuse 12 (7.5 A)	Red
Ground	Brown, 1.0 mm², through C302 terminal Y to ground G200 (under the dash above the brake pedal). Radio connector C216 pin 1.	none	Black
Illumination	Grey, 0.5 to 0.75 mm², through C302 terminal W to radio connector C215 pin 1 (radio terminal 58). This feed follows the dash dimmer.	Through the light switch and dimmer	Orange, or leave it unconnected

Fuse 12 and fuse 21 also feed other circuits (the on-board computer, interior lights and the instrument cluster), so a dead radio and a dead clock together point at one of them. Use the fuse finder to see what else shares each fuse.

If the new unit loses its station memory every time you switch off, the constant and switched wires are swapped. If it will not turn off with the key, the same thing applies the other way round.

The antenna

There are two parts to the antenna: the aerial signal and, on cars with a power antenna, the motor that raises and lowers the mast.

Signal: the antenna lead is a coaxial cable that plugs into the back of the radio. Most adapters include a plug for it. Check that your unit's aerial socket matches.
Power antenna feed: the mast motor is fed with constant 12 V on a 2.5 mm² red wire through C302 terminal Z and connector C219, on fuse 28 (30 A, shared with the cigar lighter). Its ground goes to G302.
Raise and lower control: a white wire (0.5 to 0.75 mm²) goes to the radio's power antenna output. The radio puts 12 V on it when it is on, which raises the mast. Connect it to the head unit's blue power-antenna lead, if it has one.

If your car has no power antenna, leave the blue lead unconnected and insulated.

Speakers

On the basic radio, the speakers are not connected straight to the radio. The factory wiring runs the radio's left and right outputs through a fader control (the front to rear balance potentiometers), and each side shares a common wire with its two speakers. That arrangement does not suit most aftermarket head units, which expect each speaker to have its own pair of wires and which cannot tolerate a shared ground.

The usual approach is to bypass the fader and wire each speaker directly:

Disconnect the factory speaker wires from the fader control and the common wires.
Run a separate positive and negative wire from the head unit to each speaker. Do not join any speaker negative to the car's ground.
Use the head unit's own fader control for front and rear balance.

Speaker	Aftermarket wire pair (standard colours)
Front left	White (+) and white with black stripe (−)
Front right	Grey (+) and grey with black stripe (−)
Rear left	Green (+) and green with black stripe (−)
Rear right	Purple (+) and purple with black stripe (−)

The factory diagrams show the front speakers as a woofer with a crossover that also feeds a separate tweeter. Convertibles have no rear tweeters. If you swap the speakers, fit new ones with a matching impedance, or use the head unit's built-in amplifier and low-impedance output.

Cars with the sound-system amplifier

On these cars the radio does not drive the speakers. The radio's four outputs go through the fader to the amplifier, and the amplifier drives each speaker. The amplifier is switched on by a white wire that comes from the same radio output as the power antenna. Its connections on 6500-1 Radio/Antenna with Sound System are:

Amplifier pin	Function
11	Constant +12 V (1.5 mm², fuse 27, 30 A, which is shared with central locking and interior lights)
10	Ground, to G106
13	Turn-on, a white wire that carries 12 V when the radio is on
18	Signal ground, joined to the radio ground
14, 15, 19, 20	Inputs from the fader: left rear, left front, right rear, right front
2 and 3	Right front speaker (+ and −)
16 and 17	Left front speaker (+ and −)
1 and 7	Left rear speaker
4 and 5	Right rear speaker

You have two choices:

Bypass the amplifier. Unplug it and wire the head unit straight to the speakers as described above, using the speaker wires that run to each position. This is the simplest option, but the factory speakers may sound thin and quiet without the amplifier's power.
Keep the amplifier. Feed the head unit's output (preferably its pre-amp or line outputs, otherwise its speaker outputs through a line-level adapter) into the amplifier's four inputs, skipping the fader. Connect the head unit's blue and white remote lead to amplifier pin 13 so the amplifier switches on with the radio.

Wiring it up

Use an adapter harness that plugs into the car's radio connectors, and join it to the head unit's own harness on the bench. Crimp or solder each join and cover it with heat-shrink. Twisted wires and tape are not a repair.
Match every connection by function, using the tables above. If the adapter's colours do not match the standard ones, trust a meter, not the colour.
Connect the antenna and, if fitted, the power antenna lead.
Reconnect the battery and test before you refit the trim: switch on in ACC, check each speaker with the balance control, check memory by switching off and on, and check the display dims with the dash lights.
Refit the radio in the aperture and secure it.

If it does not work

Symptom	Likely cause
Dead, no display	No power. Check fuse 12 (switched) and fuse 21 (constant), the head unit's own fuse, and the ground to G200.
Works, but forgets stations	Constant and switched wires are swapped, or the constant feed is missing.
Display dim or off at night	The illumination wire is connected where it should not be. Try leaving it off, or use the unit's own brightness setting.
Whine that follows engine revs	Poor ground, or the power or speaker wires run alongside the ignition wiring. Fix the ground and route wires away from it.
One speaker dead or very quiet	Wiring to that speaker, or one lead joined to ground. On sound-system cars, check that the amplifier is switched on (12 V at pin 13).
Whole system dead on a sound-system car	Amplifier fuse 27, the amplifier's ground at G106, or no turn-on signal.
Mast does not rise	No feed on the antenna wire (fuse 28), no output on the raise and lower wire, or a worn mast motor.
Static or poor reception	Antenna lead loose at the radio or at the base, or a damaged mast.

For a full look at how the radio and amplifier are wired, see 6500-0 Radio/Antenna with Sound System (radio), 6500-1 Radio/Antenna with Sound System and 6500-2 Radio / Power Antenna (325e).

Written from the factory wiring diagrams on this site and general diagnostic practice. Voltages, resistances and pressures are typical figures, so confirm them against your car's manual. Take care with fuel, high voltage and the battery.