Here's how Ford wired the Ex seats in 2000
BK/WH - 25A fused circuit, always hot
LB/WH - 30A fused circuit, always hot
GY/YE - 7.5A fused circuit, hot in run
Each of the 3 power circuits is from a single fuse, then split to the two seats. Be sure to use adequate wire gauge for the nominal current in each circuit.
The heater switch is usually on the outboard side of each seat, near the motor controls.