98 k1500 just swapped out motor and transmission. Now won't start. Brake lights stay on when key is on. When key is turned to acc the "P" illuminates but when you turn to on position it goes out. Also gear shifter will shift out of park without touching brake pedal. Went through all fuses and replaced neutral safety switch. I'm stumped!!!!

 

If your brake lights stay on AND you can shift from park without applying the brake then your brake switch is stuck in the "depressed" or "down" position. Your neutral safety switch receives a signal from your brake switch mounted above your brake pedal to allow you to shift from park and if it receives the "ok" signal all the time then your NSS thinks you have the brake pedal pressed down.

As far as the no start it could be a number of things but I'd start with fixing the brake switch.
Get that sorted and let us know
Also you HAVE GOT to have the main transmission plug (big green one) plugged in for the truck to start, it won't start if its not plugged in or plugged in right and sometimes its kind of tricky to get it plugged in the right way
Good luck