Ok I really don't know anything about hooking up amps.. so I have a silly question. My amp only outputs sound to the speaker if one of the speaker terminals per channel is grounded. Is this normal? I.e. if I just screw my speaker wire into the terminals no sound comes out. Add a ground to one of the two screws on the left and right channel and everything is peachy.
Mind you i bought this amp at a junk shop for $12 so the amp could be broken in some way.. but I never knew for sure if having to add a ground to the speaker terminals was normal or not.

An aftermarket amplifier should have the following connections:

(Power)
12 volt constant power from battery-> fuse block near battery-> Amp power.
Ground wire-> Amp ground wire.
Amp remote activation wire from stereo->Amp Remote connection on amp.

(Audio to amp)
Left and Right RCA plugs from stereo->RCA inputs to amp.

(Speakers)
Left speaker + -> Amp left speaker +
Left speaker - -> Amp left speaker -
Right speaker + -> Amp right speaker +
Right speaker - -> Amp right speaker -

If that does not work, either something is wrong with the amp, or there is something wrong with the wires that run between the devices (for example, the speaker wires are frayed and touching the car body somewhere). There also might be something wrong with the speakers, such as a shorted voice coil.

This amp only works like this
Left Speaker + -> Amp left speaker +, and ground
Right Speaker + -> Amp right speaker +, and ground
Left Speaker - -> Amp left speaker -
Right Speaker - -> Amp right speaker -

Tried it on several different speakers (speakers outside of cars etc..) so I don't think it's the speakers. Must just be a "broken" amp I just thought this might be some alternative way to hook up some amps.

It might be a rare amp that actually uses the car chassis as part of the grounding loop for the speakers. I've heard of them but don't know much about them.

Oh yeah, that's true. Those are pretty old, though!

That must be it... probably the reason why I got a Kenwood 40-watt amplifier for $12 too --because the guy at the junk shop couldn't get it to work and thought it was broken.