I have a 97 Passport with an after market Pioneer. It looks to have been professionaly installed. All wires have pro style caps and has a heavy duty harness. The rear speakers play but are really low toned and muted. If you turn down the front all the way you can hear them but they sound like they have 0 treble. I hooked up a spare speaker to the rear wires and it sounded exactly the same. The head unit is a new model and works perfectly. Is there something in the truck that could effect the sound of the rear speakers like a factory amp? I can of course pull aprt the wiring and swap some wires around to see if the rear out put on the stereo is bad but before I pull apart the beautiful wiring job I wanted to ask you fine people.
thanks

one of the wires going to the speakers are reversed check both leads to determin which one...most likely its not at the harness but at the speaker itself.

I pulled both speakers out and checked for that. They are factory and are only able to be connected correctly.
The harness wires "appeared" to match up.
I think my only option is to undo the harness and check from there.

also check on the radio. Pioneers have a feature where you can turn your rear speaker channels to a subwoofer output. Make sure you have not done this. If you can not find this feature reset the radio but getting a pen and holding the reset button for 3-5 seconds or just unplug the radio and all settings should go back to factory. To get to the option you usually have to turn the radio off with the key on and hold the function button and then go through the menus.

also check on the radio. Pioneers have a feature where you can turn your rear speaker channels to a subwoofer output. Make sure you have not done this. If you can not find this feature reset the radio but getting a pen and holding the reset button for 3-5 seconds or just unplug the radio and all settings should go back to factory. To get to the option you usually have to turn the radio off with the key on and hold the function button and then go through the menus.

That's one I had not concidered. I will try it today and see what happens. Thanks for the help, I let you know what comes of it.

did you ever figure out what what causing this problem?

I still have the issue. I went throught the menues and changed the out puts and it didn't make a difference.
My next step is to hook up a different unit and see if it changes the sound.
Different speakers didin't help either.

Any more suggestions? This is a fairly new head unit.