Unfortunately it's clear your original Volvo installation has been 'tampered with' so who knows what you now have wiringwise!! There would have been a simple solution originally when the Blaupunkt head unit was installed using an after market 'Autoleads' wiring adapter set.
The original Volvo head unit (CR...?) had an external amplifier situated under the dash above the driver' right side knee (UK models!). In those days Head units didn't have amps built in!! Remove lower trim and you will see an aluminum finned 'thing' (about 5 x 3 inches) attached by 2 bolts to the strength dash strut.
Originally the Volvo head unit only fed (if fitted) speakers on the dash. The door were fed from the amp with the amp being fed (L&R) from a separate fly wire from the back of head unit to the amp. Wiring then went to all doors from the amp.
I have a 1990 745 originally fitted with a CR-702 with a new head unit that runs blue tooth/hands free phone/usb/sd etc (too much for me to cope with!!) .. all with only an Autoleads PC2-19-4 stereo harness with amp bypass to update the wiring to ISO standard.
Best of luck .. Bob
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