I installed a new headunit (pioneer avh-p1400dvd) and front speakers. I left the stock rear speakers running from the head unit and hooked the front speakers up to a 4 channel amp (mb quart onx4.125). As soon as I hooked up the front speakers to the amp my rears speakers stopped working. I tried fading everything to the back and get no sound. My fronts work fine however.

any reason why this may be happening.

thanks in advance.

 

What car?

 

Oh sorry, its a 2004 ford explorer xlt

 

Can the internal amp (in the radio) be turned on/off? Chances are the default settings are set to off

 

It probably has a factory amplifier. You need to connect the blue and the black/white (if there) to light it up.