The wiring on these is a bit different than on the later ('71 and on and '73 and on) cars and here's the fuse box layout. The fuses are numbered one through ten, left to right:
|1||5A||Parking light and tail light, left; engine compartment|
|2||5A||License plate, luggage compartment; instruments illumination|
|3||5A||Parking light and tail light, right|
|4||8A||Clock; interior light|
|5||16A||Lighter, power antenna|
|6||8A||Windshield wiper and washer|
|7||8A||Stoplight, horn, back-up light, turn signal, auto choke, warning lights (man. choke, fuel, oil pressure), fuel and water temperature gauge; electrical fuel pump|
|8||16A||Back window heating, air conditioner|
|10||16A||Power sliding roof, other electrical accessories|
5A fuses are amber (yellow), 8A fuses are white, and 16A fuses are red. The fuse 7 circuit has quite a lot of components on it and when that blows you'll lose pretty much the entire instrument cluster.
Note that there are no headlight circuit fuses, this came later with the two-row fuse box. I suggested adding in-line fuses for the headlight circuits. I also recommend adding relays for the lights if your car doesn't have them (one for the low beams and one for the high beams) in order to reduce the current flowing through the headlight switch. This was also done on later cars, although it seems to not have happened until the 1973 model year.
There seem to be three ignition switch parts for the USA-market E3. The very early cars like this 2500 have a unique ignition switch style that is not interchangeable with the later styles (that is, they don't fit each others' lock cylinders). The part number for this early switch is 61-31-1-352-236 and the part is no longer available from BMW. The part number for the slightly later style (used until 04/1971 production, likely '72 model year) is 61-32-1-350-532 and the part is available. The later switches have slightly different wiring tails, including an additional key position, but they can be spliced into an older-style harness with the additional key position left off.
Replacing the early ignition switch
It's possible to replace the very early version of the ignition switch with one of the later switches, however that involves also replacing the lock assembly and changing some wiring. The early ignition switch is connected as follows:
- 12+ from the battery (red) is common to the switch.
- small gray wire with a plug connector powers the turn signal switch. This is not a specific switch position but is used for the hazard flasher and side market circuits, along with the turn signal itself.
- heavy green wire connects to fuse 7 on the fuse box. This powers the accessories and provides the "run" or "on" position.
- black wire powers the starter solenoid, the "start" position.
Replacing the turn signal stalk
The early turn signal stalk is functionally identical to the later pre-74 stalk but wiring connector is different. You should try to find the very early type what will plug right in or splice the later type into the early harness. The very early cars use a rectangular plug with rectangular female connectors inside, the later cars use a square plug with round male pins inside.
The turn signal moved to the left side on the 1974 model year and is that one is a quite different design.