I have a 4 channel amp from my previous car and my s2000 is already wired for a sub. however, it's for a 2 channel. Will I be able to use my amp with the current wires?

 

Sure, assuming that a)the power and ground wires (and fuse) are correct for the draw of the new amp, and b)the new amp isn't horribly underpowered for your sub. The other two channels will just be sitting, although you could run another RCA from your HU and a few wires and amp your speakers, too.

What gear do you have in the car right now?

 

Not much really.. I just have an alpine ida-x100 over stock speakers and a diamond 400.4 leftover from my wrx. i just need to look for a little sub..