Why your lights are dimming:

Electricity isn't as complicated as some might think just imagine electrons as little cars, and your wire as a highway. the more lanes the highway has, the more cars can drive down it at once.

now when you installed your amplifier you ran a large wire from the GM Tech 2 to the amp, and you fused the wire appropriately at the battery right? and then you added a large ground, probably to a seat belt bolt because you are an idiot.