I am having trouble getting my radio to work after swapping the alternator out on my 1996 850. After replacing it with the one from my 1994 850 parts car, everything seems great (charging right at 14v) but the radio won't work. It does not display anything: no "CODE" or "OFF". However, the little red antitheft light blinks like the car is off (and slightly dimmer if I'm running the car with the lights and a/c on). I have tried the stereo from my parts car and it does the same thing: nothing. It's almost like the radio doesn't know the car is on. I have checked fuses to no avail. What could be causing the radio to not recieve the signal to 'wake up'? Is there a relay I need to check somewhere? Any assistance would be greatly appreciated.

Okay. I checked which pins in the harness were getting power last night with a test light. According to the diagram I have, pin #5 is the "power antenna turn on". It and the pin immediately to it's left in the diagram are the only ones getting power. They get power with the key in all positions. No other pin gets power at all, no matter what position they key is in. It also appears the ground wire is not grounded as when I attach to it and a known power source, I get no light from the test light. Anyone know what's going on here?

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When you say "immediately to it's left", I am assuming you mean pin #3.
http://www.tehnomagazin.com/Auto-radio-car-connector/VOLVO-Car-Radio-Wiring-Connector.htm
The cast of characters (others not listed are speaker wires):
#1 - +12V ignition
#2 - Ground
#3 - +12v battery
#4 - Dash light dimmer
#5 - Power antenna turn on

Pins 1, 2, and 3 are the important ones to check.
#3 should have power at all times.
Use a VOM to check your ground wire (pin #2) to make sure you have continuity and do not have high resistance.
I suspect that if pin #1 does not have power with ignition key in position II, that is your issue.
--
Optimist - half full, Pessimist - half empty, Engineer - The capacity is twice what it needs to be.

SOLVED! Sometimes it seems the simplest problems are the hardest to solve. It turns out, when I was checking the fuses (repeatedly) I apparently sprung the radio fuse contacts out. They were not making good contact with the fuse blades, thus not allowing power to the radio. So, I used my pocketknife to spring the contacts together a little and what do you know, the radio is back to enlivening my commute. Hopefully this info can help out someone else if they experience similar difficulties!