Welcome mate!
In regards to the amp/DVD player, is it a 7 inch touchscreen one? or??
Check all fuses on the amp, inline fuses, check the ground and positive connections as well as the remote (comes from the headunit to turn the amp on when power is at the headunit, usually is a blue cable) Also be sure to check the RCA cables and see if they are connected properly.
feel free to PM if you need more info
If all else fails, take it to JBHIFI and see if they can diagnose it
best of luck, and welcome to the forums again
Brett