1994 Grand Voyager LE diagrams

This one is a toughie as these cars are kinda different from year to year.  Really the main problem that you're gonna run in to is that your head unit does not have speaker-level outputs and thus to make the phsyical connection to the factory amp is not easy.  You could solder rca plugs onto a standard chrysler/plymouth harness, as long as you ground them properly.  However, if this infinity system uses a common ground system then you'd need to use an adapter like the Peripheral CHYAH in order to properly interface with the factory amps.  I don't know if this adapter will work if it is a standard (floating) ground factory amplified system.  Might wanna try to email peripheral and ask them.

Usually when we are doing head unit installs on these chrysler cars with the infinity systems, all we do is hook up the aftermarket head unit's speaker wires up to the chrysler harness wires as if it were a standard stock system.  As long as you do not turn the volume all the way there is nothing wrong with hooking it up that way as the factory amps use a high-level input anyways.   Once you start to send too much wattage to them, however, is when the sound starts to get distorted.  But I have yet to see this happen on one of these vehicles, I just know that it's technically possible.

With that being said, we come back to your problem now which is that you're trying to go from low-level to high-level, and if you think about it that's basically what an amplifier does.  I saw this harness at installer.com but not only could I not find it anywhere else, but it says "coming soon" so i dunno what that means.  But still if you could get a hold of one of these or if you are good enough at electronics and soldering to be able to make one of these, then I bet it'd work.  It's the 70-1818 which makes it sound like a metra part number, but I couldn't find it on metra's site either.  Try emailing metra explaining your situation to them--they usually respond pretty quickly and should be able to tell you exactly what to get.

http://www.installer.com/wires/70-1818.jpg

I think that's part of the fun of it--every car is different so you're always doing something different.  And then add to the fact that they come out with new cars every year; so you're constantly being challenged (well, to some extent).  Sometimes it is kinda annoying when we see manufacturers use the most illogical wire colors and stripes and stupid stuff like that--as if it's less expensive to make the harnesses that way than to just follow the EIA wiring colors? lol

You might need a line driver to raise the preamp level to a high-level type input that would be compatible with the factory amps.  Or an integration adapter might be needed...it's kinda hard for me to tell since i'm not there with the system in front of me, etc.

You are correct about the high-to-low level converters.  That's pretty much all there is out there because, well, basically it comes down to this:  most people who buy an eclipse head unit--especially one without a built-in amp, will also have all aftermarket amps and speakers.  Hardly anyone will use a head unit with no built-in amplification, with a stock amplifer system.  I'm not saying that what you're trying to do is wrong or weird, because it's not at all.   I've seen it done before and done well.  But it's not what I would recommend.

Just remember that stock amps can't give you the sound quality of an aftermarket 4-channel will; and this will become even more apparent when you have aftermarket speakers installed.