Marc, I'm not clear from the pictures how you are getting the power wires up to the location under the dash. Are you running them through a starter solenoid first, and having the solenoid act like a switch?
If so that might be a problem, because if I recall correctly the starter solenoid contacts are not rated for continuous operation. In fact, I think I may have actually tried that years ago and had problems; can't remember its been so long...