2002 gmc sonoma won’t start

Ok so me and my buddy are instaling a amp and sub and head unit in his 2002 gmc sonoma. Yesterday he had everything besides the wiring harness so we unhook the battery then take out the dash parts take out the stock head unit, run the power cord from the battery thru the firewall to the back of the cab and hide it and all (we never hooked it up to the batery) then ran the rc wires down the other side (we left them just siting at the back of the stock head unit). It got dark out so we figured wed finish up next weekend when the wiring harness comes in. So we put back in the head unit, then put the dash back on. So he starts up his car and drives home. This morning he went out to start his car and nothing. so we jump the batery and it starts. We let it idle ther for miniute and reved the engine and ****. So we go to pull out of the drive way and the car shuts off. Wego to try to start it again and it wont start. What do you think i should try and check out?
any help would be great.
Thanks
Kevin

Check your battery connections. I know it sounds dumb, but the smallest things are the important things.