Matt,

The AMP-lights in the dash do always light up slightly with all those "damn old volvo's" when wipers and/or blower is turned on. Sudden flickering however would indeed indicate a short or perhaps ground problem. (not the belt)

When even a radio cuts out due to lack of "juice", voltage must be very low, since they hardly need any current to operate: if it realy was the battery voltage, your car would have stopped running aswell: it wouldn't have anough power to ceate a spark.

Check grounds and contacts, but first try disconnecting the radio itself, perhaps he (or is it a she) is shorting.

Good luck, ben