My present configuration involves an empeg player, a two-channel amplifier, and front/rear speakers on each side connected in parallell to the lef/right outputs of the amp.
My upgrade path looks like it'll be

1) 4 channel amp, or a second two-channel amp.
2) A subwoofer
I'll probably buy a "self-contained" sub which I can have in the trunk, and just remove when I need the space.

My question is: How do I mount the Sub?
Can it be connected to *both* the left and right rear speakers? Connected to only one of them? Add a separate amplifier? How is this amplifier connected to the 4 outputs of the empeg player?

Any suggestions?

Marius (Escort Cab + Mark II)

All of that is taken care of by the subwoofer amplifier. You run a left+right pair of RCA cables to the subwoofer amplifier, and it handles the messy details of combining the stereo signal to get a good mono signal out of the woofer.

Diagrams of the different ways you can wire a subwoofer into an empeg system are here.

Thanks.
What I really should have asked was "Can I use Y-adapters", and it turns out the answer is "yes" ;-) There is no current/impedance problems?

Marius (Escort Cab + Mark II)

I have been told that there can be current/impedance problems. But I didn't have any when I wired it the way it was diagrammed.

Note that you can only do this stunt when splitting the signal. Don't try to use a Y-adapter to combine signals.

By the way, purchasing an amp with pass-through connectors is highly preferable to using Y-splitters.

True, I think a good idea here would be a 4 channel amp (one channel per speaker in the car), with a pass through output to go into the 2 channel amp (to be used for the sub).

This would assume the 2 channel amp is powerful enough for it, and can be bridged as well, but that's another story.