Sounds like a wiring problem. The cruise control is designed to dis-engage if you tap the brake thereby sending a signal electricly to it. It must be wired into a common power source with the wiper control. Something from the wiper power is telling the cruise control that you are stepping on the brake and to dis-engage.
If you can hit the cruise reset and restore the cruise control, that would be a way to check my theory. If it turns completely off, then it not the brake signal but maybe in the switches.
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