Is it possible to just change the amplifier and not the head unit? I have the Mitsubishi infinity system with 4 disc changer, but the amp doesn't work. Does anyone know what the ohms are for the speakers for this system?

You should be able to change the amp. Depends on where it is located.

coupe or sedan? you said mitsu, so i am guessing that you have a 2g coupe. if so, the stock amp is under the passenger seat. i think that 3 or 4 bolts hold it in place. the bigger issue with just replacing the stock amp is that there is a harness going into the stock amp, and a harness coming out. so you would need to either rewire things, or make a pigtail.