Hi everyone,
I have opened up the space under my dashboard behind the steering
wheel ('91 535i) and found a way to snake wiring from the cabin to the
engine compartment.  The easiest way is along the tube (piggybacking)
that goes from the hood release lever to the hood release latch in the
engine compartment.  It appears that a small hole has been drilled
either through the firewall or along the edge of it by BMW at the
factory to allow for this lever line to run from the cabin to the
hood.  The hole is wide enough to get wiring in alongside the tube
that holds the hood release lever.  Through the hole from the engine
compartment just behind the fusebox and into the lower dashboard
behind the steering wheel mechanism goes the wire.  In addition, for
the benefit of others who may need to know:  I have identified the
colour schemes for the ignition switch on this car.  The red wire is
hot (straight 12 volts from the battery positive), the green wire is
the accessory, and the purple wire is the ignition wiring.  I assume
that this purple wire goes to the starter solenoid and from there goes
to the distributor, rotor and spark plugs.  If you need a ground
connection under the dash behind the steering wheel, just look for a
bunch of brown wires stranded together and all going to a nut screwed
into a bracket.  This is your ground connection, just unscrew the nut
and loop your ground wire around the bolt and screw the nut back on
tight.