I vote for a FULL re-wire and take pics of your set-up so we can guide you!
Wiring an amp and sub is virtually plug and play! Just have to make sure you Check the correct ohmage of the subs or you get nothing

If you don't want to re-wire;
1) Could the power lead be getting pushed out when sliding the HU in and out?
2) Is your battery strong enough?! (Silly question but easy mistake!)
Hope i helped a tad

Jon