Look on page 21-20 of that manual. Pins 1 and 4 in the relay box should have 12V to a good ground regardless of the pressure switch. The pressure switch will not cause you to have 3V on one of the pins.

What are you using for your volt meter ground? Are you testing the fuse box sockets with the relay unplugged?

To determine if pin 1 or 4 has 3V, can you tell me if the higher quality metal plugs into the 3V pin or the lower quality metal? That will determine where to troubleshoot next. Alternatively, you can pull fuse #20 in the under hood fuse box to see what goes missing at the compressor relay or fuse #16 in the drivers interior fuse box.