There's some useful car audio and general wiring info at: the12volts.com

I recently installed an Aiwa unit in my 86 240 - made easier by the fact that the previous installer had figured out the matches between the non-standard Volvo wiring color-code and the wiring color scheme that all maunfacturers seem to use these days.

When I first turned up the volume the head unit shut down. Found out that the previous Technics cassette player had a 1 amp in-line fuse on the yellow wire now used for constant 12 volts. New Aiwa unit has a little transformer and some electronics in an in-line box with a 10 amp fuse on this same line. 1 amp fuse had to go.