[legion]

Yes, the alternator is probably bad. You should be at 14 volts, maybe a little less when blasting the stereo with the headlights on. I put an aftermarket voltage regulator on my old 951 and set it at 14.5 volts when I had it rebuilt. This solved a lot of quirky electrical problems with the car.

You probably also want to replace the main grounds along with a new/rebuilt alternator.

I'd also check for voltage leaks. Do a search for posts by Iceshark. He had good procedures for doing this.

Also, just because a battery reads 12.5 volts doesn't mean it isn't bad. A battery can still produce good voltage without delivering enough amps. The simple way to test this is to buy a device that tests the density of the fluid in the battery. You can pick one up at any auto parts store for under $10. Assuming you do not have a maintenance-free battery (this won't work if you do), you just pop the cover on the cells and suck the fluid up into the testor. A little dial with tell you if the fluid is good (can deliver enough current) or not.