Need help with some gauges

[Camacorn]

Hey all, when I purchased my truck, it had an aftermarket oil pressure gauge, but I'm all about factory, and I want to hook up the original gauge. I'm not sure what exactly I have to do to get it hooked back up.

I'm also trying to get my Voltmeter gauge working. I tested the back of the cluster, and with the key on, the Voltmeter prongs on the back of the cluster get power, and have a good ground ( is it just a bad gauge? ).

Any help would be appreciated.