Behind passenger side head on the block, where bell housing bolts to block.
Picture here in first post: Replacing the Cam Sync
Shows engine from the back, out of the truck, to the right you will see the oil pressure switch with white connector.
This switch is either open, no pressure, or closed, more than 5psi in pressure, it is the Ground for the gauge, should just have 1 wire, probably dark green.
With key on, engine off, if you Ground this wire to the engine then oil pressure gauge should go up to it's "normal" position, if it does that means wire and gauge are OK, replace switch.
If it doesn't then the problem is most likely not the switch.