2001 Volvo S40 Stereo Wiring

You can try a Metra harness and see if that works.  If not, you will have to hardwire it.  I had to hardwire an old Mercedes for a customer, and I told him it'd be an extra $75 if he wanted to fade front to back since the plugs on the back of the radio only had common left and common right leads.  I suppose these went to an amp somewhere (I don't know), however they still "pop tested" out okay.  I hate working on German cars.  He didn't want to pay extra, so I combined the two left ouputs from his new headunit, and combined his two right outputs from his new head unit- it worked great.  You probably don't want to do that, but it may give you an idea or two.

The thick black wire is probably going to be your ground, so test this with your multimeter.  The thick red is probably constant, so again, test this.  Now using your meter, find the accessory wire.  Now all you'll need are your speakers.  Test for continuity on your candidate leads.  If nothing, you may have to search for an amp.  Hopefully this'll get you on the right track.