UniAirSusp

It sounds like you have quite a few more issues. Holding the switch open won't damage the valves, they only pull 8 watts (0.67 amps) and they are 100% duty valves (they can be left on for the life of the valve).

I would suggest taking a look at the compressors. With the compressors running probe the wires and see what the voltage is at the pumps. If your having a big voltage drop, its going to cause the amperage to go way up and cook the wires. With the GS's you may want to entertain the idea of running another ground wire back from the battery, and sandwitch it with the body ground and the ground for the compressors. We have found in parts of the body, even though you have a good connection, the body itself doesn't have a good ground.

Edit: Forgot to mention, if you have the wires from your valves to the switch box shorted out, it will cause it to pull alot more current (direct short)