1999 mustang stereo stumped.

So installed new speakers in a 1999 mustang. Passenger side both front and rear speakers are not producing bass. driver side (both) are doing it just fine. When I connect the speaker wire from the driver side speakers to the ones on the passenger side they produce bass so its not a speaker issue (speakers are brand new and work perfect when ran from driver side wires). What could it be?? a ground issue??? wiring behind the stock stereo/cd player?? any ideas would be great.