How difficult is it to replace the wiring harness in a Toyota Celica?

not as hard as you would think! the harness starts right behind the glove box. through the firewall. then you have to unhook the : battery, starter, alternator , fuel rail, water housing, and a couple of wires off the back of the block and the transmission.

i would unhook a couple of the motor mounts before getting started and jack the motor up a little.