A/C not working; need troubleshooting help on 1991 Nissan Pickup (D21)

My A/C doesn't work on my 91. The on switch does illuminate on the controls but the compressor doesn't engage. According to the last owner it was charged but had no affect. I'm suspecting electrical issues and need to know which switches to start checking for, as well as locations of said switches.

I will use this truck to commute about 8 hours on the weekend for a while and could really use it on long trips.

Asked by for the 1991 Nissan Pickup (D21)