radio tahoe aftermarket

question. I bought a 96 Tahoe and the radio in it will just quit all at once and not come back on for a few days. So i took out both the head unit and the cd player and changed it out with an aftermarket. Well I get the same thing. So my question is, there must be an external amp somewhere in...

my radio stays on when i take key out & open the door... this is an aftermarket radio.. it just started happening when i re-installed the head unit.. any suggestions???? 2007 tahoe