Ok i have installed a pioneer DEH-P4400 head unit, new pioneer 4x6 speakers and new Sony Xplod 4x6 speakers. all wired up with 16 gauge Monster cable. My main problem is they start to distort bad before i turn them up loud but i have just noticed today when i start to turn it up loud the lights on my deck start to flash and dim at like volume point 13. I have had this problem with other decks i have installed and it was just a ground problem but on my GMC here it isnt a ground problem. So if anyone can maybe help me with my problem here and maybe tell me what to look for that would be great.