marybeth,
Welcome to FTE and the IDI diesel forum.

The thing is, the glow plug system draws 200 amps.

That must be a big switch to handle 200 amps in the cab or if not the switch is what is dropping the voltage.

A relay is the only way to switch glow plug power.

What is wrong with the original relay/controller?
Or how is the glow plug system acting when it is run with the stock controller?