OK - I see now.
Definitely try and see if you can drive the two front speakers alone - could have ended up with a bum amplifier section - not completely unheard of. I've had quite a few units be bad, right out of the box, a large percentage usually do to shipping damage.
Trying to run 4 speakers from a channel headunit - might be possible. Chances are, the headunit cannot drive that impedance of two speakers wired together like that. You can wire the speakers in parallel or series - parallel will generally drop the overall resistance of the circuit (will halve the impedance of a single speaker, assuming that the speakers all have the same impedance). Hook the speakers up in series - the impedance will be the sum of the speakers in the circuit. Most headunits will put a rating on what they would be able to drive - impedance wise.
If you are trying to drive OEM speakers - this might be possible - assuming that you really have 40w a channel. Some manufactuers list P-P or peak-to-peak power, which looks good on paper, but means very little to drive the speakers. Especially if you reference the speakers to ground - given 40w p-p, you are looking at around 14w total per channel. Could be even a bit less. I believe my OEM headunit (Pioneer or Fujistu-10/Eclipse) was rated at 10wx4 continous or around 30wx4 ot 40wx4 peak.
I tried hooking up just the front left and right speakers and didn't get sound. I took one of the front door speakers out and hooked it up directly to the head unit. That time I heard the radio loud and clear. I hooked up the positive speaker wire and negative ground wire to that speaker. When I use the harnesses, I hook up the negative ground wire from the radio to the negative ground wire on the aftermarket harness.
My factory radio runs all 4 speakers. I bought the two rear and front seperately. The original speakers were old and worn out. Does that still sound like a problem with impedence? The front speakers are just replacement cones. The rear are from Best Buy and were on clerance, but a named brand (can't remember the name).
What should my next course of action be?