A head unit amplifier circuit uses a different type of amplifier setup than a power and on its own. The amplifier in a head unit already uses a push-pull amp setup (bridged) to get the amount of power needed out of such small units. As Walkinshaw said, you would blow your head unit amp if you attempt to bridge the outputs as it would put a huge load onto the circuit. Also, if it didn't blow the amp, it would sound shocking and there would be no benefits anyway cause neither wire is at ground potential i dont think. I could be wrong, but that is pretty much how they work.
i could go into more detail, but i think you can get the general idea from whats said already......
Hope that helps....
EA Fairmont Ghia : 3.9L MPFI, 2.5" cat back sports exhaust, K&N panel filter. Pioneer head unit, pioneer 4" three way fronts, Pioneer 6" 3 way rears, 2x 12" subs, blue interior light.
My main pride and joy : '76 XB Fairmont GS, 250CI, 140,000 Km and all stock.... for now! (Factory air-con on the way)