Very strange...I can't think of anything off the bat except check your wiring. How were you connecting the speaker terminals? What is the impedance of the stock speakers? What about the aftermarket speakers you tried? Were they new? Did you make sure they worked in another vehicle?
The speakers work, the guy said the factory unit plays jus fine.
My question is, do the aftermarket headunits power on? If so, they could possibly have amped speakers, or like my buddy's GA, it could be that you have to keep the headunit and just relocate it to use an aftermarket unit. If the aftermarket unit works, run a set of wires from the headunit to the rear speakers and see if they work like that, if you don't want to run a set of wires all the way through the car you can splice into the speaker wire.
Constant 12V+ = Green/Yellow
Switched 12V+ = Yellow/Black
Ground = Red or Black
Illumination = Blue/Red
Dimmer = n/a
Antenna Trigger = Orange/Blue
Antenna = Right Front
Front Speakers = 3 1/2" Dash
Left Front (+) = Orange/Green
Left Front (-) = Blue/White
Right Front (+) = White/Green
Right Front (-) = Green/Red
Rear Speakers 5" x 7" Rear Deck
Left Rear (+) = Pink/Green
Left Rear (-) = Blue/Orange
Right Rear (+) = Pink/Blue
Right Rear (-) = Brown/Red
A little information to help you out figuring out what color wires the speakers are. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif I hope I've helped a little bit.