1995 Dodge Ram Rear Speaker Replacement

The head unit is an infinity-made one I believe, and has some (but not much) self-amplification.  I'd say probably less than 5 watts RMS, but it does send out a high-level signal to the infinity amplifiers.  When i say "high level" I'm talking about a signal that's already amplified and thus is capable of powering a speaker by itself.  The infinity head unit's power output is probably about the same as that from the standard system.

That being said, it's theoretically possible for you to scrap the infinity amps and speakers, and replace with all aftermarket equipment and still have it powered by just the Infinity head unit.  You'd just want to make sure that when you hook up the aftermarket speakers you get the polarity correct.  Use a multimeter and test for voltage when the system is turned on:  positive voltage means the red test lead is touching the positive speaker wire, and negative voltage means the black test lead is touching the positive speaker wire.

As long as the front ones are still being amplified, I would think that they would be a lot louder than the aftermarket ones that you are planning on putting in the rear.  As long as both back speakers are the same and both fronts are the same, you can always adjust the fader towards the rear a bit to balance out the volume.

I don't really know what it would sound like if you were to try and use the front infinity speakers without their built in amps, because I don't know what their impedence is.  If they're like bose speakers then they'd have too little resistance and could possibly damage the infinity head unit if you try to power them just using that.  This is why I said you're best off replacing all the speakers/amps with just aftermarkets.

Before you start messing with the fronts though I'd determine the two speaker wires for each back speaker, and hook up the aftermarket speakers to the wires that lead straight to the head unit, and see what it sounds like.