Rep Power: 0

I removed a car stereo that I was going to sell, but ran into a few problems. I removed the trim, gear trim, center console trim, head unit, but couldn't access the other end of the wiring harness adapter. Also, the antenna adapter runs down the behind the head unit into an area where I can't reach and have no clue on how to pull that all apart since I haven't seen any videos on it.

 
Rep Power: 292

You should be able to take the head unit out, unplug the wiring harness and antenna from the back of the unit.
Once that's done, you can yank the harness out until you reach the connection point between the aftermarket adapter and stock harness.

 
Rep Power: 195

As Kenny said, unless they zip tied everything down, the harness should be loose enough and long enough to reach the front opening. Same with the antenna adapter.

 
Rep Power: 292

That is, unless whoever installed it decided to cut the stock harness and solder the aftermarket harness on.
I've seen it before.