Audio Systems: Car stereo in 93 Sundance, plymouth sundance, infinity system

Question
I have a '93 Plymouth Sundance Duster that came with an Infinity radio.  I also have a JVC KD-AR560 MP3 CD unit that I had installed in my previous car and wanted to install in this one.  I purchase a wiring harness and as far as I can tell all the wires are matched up correctly, but the sound is terrible.  What I get is very tinny and shrill, and I have to crank the volume way up (around 40) to be able to hear it well.  

I do not see any external amp under the passenger or driver seat...I had heard that some Infinity systems have an external amp that must be bypassed when using other aftermarket units.

Any ideas what else I can do to get this JVC unit installed and functioning properly?  Many thanks!

Answer
Without seeing it in person:
For the most cost effective resolve I would say you need to replace the speaker wiring and probably the speakers too.  The system is 16 years old.
Get rid of the entire, aged Infinity system and get used to it sounding good!
If money is tight, just put in two speakers for now, but DO run new speaker wiring from the new headunit to them.

Sadly, there is not much more you can do unless you want to try and use that amp. They make kits to splice into the amp wiring and you would probably be better served to ask a rep at www.crutchfield.com. Realize that once you do all of that, you may need new speakers anyhow.

jm