[lincolnmaster80]

One way to do it is get a continuity tester to each speaker terminal and connect one end to the one in the dash. I believe on the speaker there's imdented marks which tells you which on is possitive and which one is negative. You'll probably need a friend or your wife/girlfriend to hold the one end in the dash. While you have the speaker out, you can also identify which speaker wire is which.

Unless they changed it in '91, my 1990 750 the speaker wires are all subtly different from positive to negative. The only similarity is that on the 8-speaker system, two speakers in each corner run off the same color wire.

On the 10-speaker system, the speakers should be running of an amp which is located in the trunk/boot of the car on the left hand side (looking into the trunk/boot from the rear) behind the panelling, in which case, the wires connecting the speakers to the dash may or may not be the same and you may not get continuity as the amp is turned off.