was the factory amp removed? On my car when i wired in the front speakers (prem sound) the door, and dash speakers run off of the same channel on the deck. the amp was only a 4 channel amp so the door and dash speakers should share 2 channels from the factory i Bypassed the amp and hooked the fronts up to only 2 channels. Meaning i hooked up L +-, and R+- and had 4 speakers. There is a crossover under the dash. yours should be similar. Finding the wires for the prem sound system was a royal pain because i could not get a wiring kit to fit. If its possible i would just run new wires, its probably easier.
Dr.