I definitely agree with checking your battery and, especially, your battery terminals, first. The battery cable ends are notorious for not tightening up enough on these Nissans. Another possibility is a bad starter solenoid, which is part of the starter motor assembly. Sometimes tapping on them with a hammer can get them working during a no-start situation, but it's not the most accessible part on these trucks! There were some PNP switch issues on R51's, which is part of the transmission TCM/valve body assembly, but they usually set a trouble code. If your CEL is "on," have it checked for stored trouble codes and see if there is one for the PNP switch. For more trouble-shooting, go to the "service manuals" tab above and look up the factory service manual for your Pathy. There will be a section for "starting and charging" and it will have step-by-step diagnostics and wiring diagrams.