Often times, the harnesses would last a lifetime, unless the Jeep was subjected to salt. Sea salt or road salt from the Midwest, would deteriorate the harness. Good wire is expensive, but worth every penny. Your expertise on wiring boats was worth every penny as well.

The oil pressure gauge you are using is the original gauge you had when first starting the engine after rebuilding? If nothing else has been changed except for the sending unit, then the sending unit is retaining pressure when none is applied. If the gauge wasn't used until now, then the gauge itself may be hanging up at 45 psi, and won't go down to zero.