1996 ford stero wont turn on

got a 1996 ford F-150, and i have gotten the old head unit out and got a wire harnesse and have it all hooked up but wen i start the truck it dont turn on...though it was how i wired it but the i tried the old head unit and it didnt work either. any ideas?
If you have the unit out, grab a test light & check for power on the harness. It should have at least 12v on one of the wires....I don't have a schematic handy but at least one wire should have 12v with the key on. You can check the harness with the key off & if one wire has 12v that's for memory, turn the key on and there has to be another wire that has 12v, that would be for the deck power. If you have power on 2 wires, then you need to find the ground wire in the harness & verify that it is good.