Been awhile but....

You probably have an amplifier! It's probably located above the pedals, under the plastic panel.

The speaker wires will all go to the amp. It will have a power line and ground also. You will have to run these speaker wires to your new head/amp. I had to extend mine to reach.

Easy way to figure out the speakers, which is which, is to get a 9 volt battery, kind you use for smoke detector or radio, and connect suspect speaker wires to the terminals. They will make a noise when you hit speaker ground and the plus side wire of each speaker. Your radio (oem) will have wire for antenna, power (switched), power (constant), and ground. I don't recall a variable voltage wire for dimmer!! It might be there though.

The schematic from new radio will show how to connect your new plug to which wire. There is a wiring color code that I've seen somewhere on the brickboard, but I can't remember where!

good luck,
jc

I
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1985 740gle na, 1990 740gle 16 valve