Soooo, my friends car has a weird system in it. the speakers are shared on a common ground, fl, fr, rr, rl, all have the same ground, seperate positives. Is there anyway to hook up another head unit to that wiring system? When i tried with the head unit he bought (Alpine CD-9847) i took all the negaitves from the headunit and put them on the shared ground that the cars system had already, and the head unit just kept cutting on and off. I didn't wanna fry it, and i've never seen this before. So if anyone could help me it would be much appreciated.
The older radios had common ground speaker returns. The returns were usually connected to the body ground, though not always. These radios could be categorized as "low power radios". Almost all modern radios are in the "high power" category.
The high power amplifiers require separate wire pairs. There are usually 8 wires, 2 for each speaker. These wires must be independent of all other wires and each other, or the amplifier chip will overheat and fail.
All 8 of these wires coming from the radio amplifier have a DC voltage of ½ battery voltage at all times. That is about 6 volts. If any of these wires are connected to ground, the radio is stressed immediately even with the volume turned completely down. When the volume is increased the AC audio voltage increases and even more thermal rise occurs. Do not do this.
The only reason you have not burned out the audio output IC already is because most of them have internal current sensing and shut down protection. These protection circuits will not prevent failure from continued overheating.