I ran into this exact same problem with a 1991 plymouth voyager van i had. I replaced the factory infinity head unit with a pioneer 4100, and i had to purchase a signal converter to change the regular output that the pioneer sends out to a line-level output. I had a problem with the signal converter, though. The pioneer put out too much power to the converter the stereo shop sold me, and it fried it and caught part of my dash on fire. This brings me to my next suggestion. Just do what I did. Yank all that factory Infinity junk out (it's not real infinity...it's chrysler infinity. it *****.) and just run new wires and install whichever pioneer or real infinity co-ax's you can afford to put in. it will sound way better, and probably even a little louder.
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