Check the speaker wiring. You should have a 4 channel amp on that (4 speaker outs) make sure all outputs are used and that no 2 speakers are "wired up" together, that would drop the ohm rating to a lower level causing the amp to attempt to double its output power(they don't always do) and when they reach a certain level and try to pass too much current, the protection circuits kick in and shut it off. However it "dimming" out on you when you throw an ACC switch could mean that it's not getting enough voltage or there's a short somewhere in the system as well. Start easy first. Look at the speaker wiring, make sure all outputs are used and that no 2 speakers are touching. Now visually inspect the Power supply wires as well. Look for cuts and crimps. Check all connections too. When you turn on an ACC, there's a quick tax on the electrical system. The head unit being a sensitive electronic machine that is, could shut off when the voltage goes below a certain level. Try a couple of these things and get back to me.