When I built my 1st gen I used the head unit from a Starion, I had bought a full running 3 year old car to get the engine.
Many Mitsus have stereo controls in the steering wheel. Way back in the 80's they used an inferred transmitter in the wheel with a receiver mounted in under the column switch assy.
The Montero just needed trimming out some plastic on the dimmer side to mount the receiver, this has a light 4 wire cable that plugs into the OEM radio.
The transmitter in the wheel gets its power from the horn circuit.
Newer units use the clockspring for the steering wheel controls. I should look into using a steering wheel from a 3000GT or a Diamante to control the headunit from a 3000GT that I use.