amp and sub installed...engine noise comes on ONLY when headlights are on...any ideas?

 

your power must be getting interfernce from the lights, if you run zenons that alot fo jiuce.

 

My buddy just ran into this problem not too long ago. Try checking all of your grounds. The best ground is your key n the ignition. Try touching it to everything that you just installed. Run a piece of wire from your key to whatever you installed. Because it sounds like a bad ground.