I might be the "one guy" Onkel Udo is referring to. I adapted a 40W+40W rear deck amp from a late-80's Volvo 760 to the Blaupunkt stereo's preamp outs in my daughter's 940. It added a lot of oomph to the Infinity Kappa 6x9's in the rear deck.

I got the correct DIN connector from an electronics supplier (Digi-key) and soldered the 16-ft twin RCA patch cords with the turn-on lead from the Blau to it. I found the pinouts for the Volvo amp online here somewhere, and confirmed by removing the amp cover and inspecting the interior circuitry. It was a fiddly job, but the results were very satisfying. The Volvo amp cost me about $10 from the PnP.

This was about 6 years ago, and all my notes and sketches have been well buried (or lost) in a house move so I can't offer specific help. I believe those adapters offered by Crutchfield, Scosche, et al, only connect the high level (speaker) outputs of aftermarket stereos to the Volvo speaker harness, bypassing the Volvo amp. I have never seen an adapter to connect the RCA preamp outputs of an aftermarket head unit to the line-level DIN input on a Volvo amp.

Would I do it again? Well - no; I'd buy a decent medium power amp with RCA inputs - much simpler.
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Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.