Truck wont start/ Some weird issues now

[LongrangeLauber]

So I did a tranny and engine swap in an 02 Silverado over the past couple of weeks, that way I'm not rushing myself and making a huge mistake (5.3L and 4l60e). I keep having problems with the Backup/ Neutral Safety Switch. I've gone through quite a few and I'm going word for word from the installation instructions, but I'm getting zero positive results from this. I install it in the neutral position, just as the instructions say to do, along with the tranny in neutral and the shift cable+lever in the neutral position. I finish up, hook up the ground, and try it out. Nothing. The instrument cluster does not illuminate the park symbol, but I go underneath the truck and the cable and lever are all the forward, where park is. It won't even let me start the vehicle, my guess is because the computer does not know it's in park, even though it truly is. Also, when I do turn the key to start it, all the lights on the instrument cluster and on my head unit go out, even where the odometer and gear selection are. Can someone tell me what I'm doing wrong? Ive messed with it for about a week, and cant figure this thing out. If I can't get it to work, then off to the dealer (somehow, not sure how I would get it there since it wont start) it is. Thanks

[LongrangeLauber]

It does start in neutral, which is drive on the instrument cluster. Instrument cluster layout is the standard: PRND321