Sounds like you might have the amp remote output (blue I think) or perhaps the power antenna output of the empeg connected to the dimmer line of your car. Basically something that's switch 12V on when the empeg is plugged in (possibly only happens if the empeg is on?)

When you plug the empeg in, it forces 12V on the dimmer line making everything go full brightness and overriding what you have for dimming in the car.

Does your car have a headlights on line and and dimmer line? If so, you don't need the dimmer line at all, just use the lights on line.

Double check your wiring I think....