Audio Systems: Infinity stereo system, dodge stratus rt, alpine cda 9857

Question
I have 2001 Dodge Stratus RT with Infinity system. I've been told back amp went out-sounds like AM radio not tuned in, front sounds okay. I replaced head unit with Alpine CDA 9857 and was told by Bestbuy techs they oould bypass the amp. $700 bucks later I have same problem and am now being told I have to replace amp with Dodge factory equipment or replace speakers and wiring to to ohm mismatch. Can I replace the amp with after market? Any suggestions?  

Answer
You sure can!
For the best sonic results, it will require all new wiring, and probably speakers, but it is well worth the trouble to get that '01 into shape, mobile sound-wise. Few things are more frustrating than most half-baked upgrades turn out to be.