Where is the Neutral safety switch located?

It is inside the transmission as part of the valve body control assembly. To get to it you need to pull the transmission pan like you were doing a fluid swap. Then you remove the filter and the valve body and take it apart to remove the control. The attached shows the process. You only have to go that far to replace the control plate, then you put it back together and fill it with fluid. However, because it still starts in neutral but not park every time, I doubt it's the switch. In this case the switch is a simple magnetic switch, a magnet moves over it in park and neutral to make it work. So, if it works in neutral it should work in park. However, it may be that it doesn't fully go into park if the shift cable was stretched, or the bushings are worn. It might go just to the point where the parking pawl engages. There are plastic insert bushings on each end of the cable that can wear. I would check those for wear and damage. They just pop off the studs they mount on. The cable can also be adjusted some if needed. To see if it's the cable the easiest way is to go under the truck, remove the bolt and the cable then make sure that the transmission shift lever on the side is pushed all the way toward the rear. Now try to start it if it starts the cable, is the issue. The bushings are easy to replace.