All WPC95s shipped with a 9-pin, somebody robbed yours. The switch for it plugs into that 3 prong harness you are seeing and the power is supplied through a black and white cable typically spliced into the main power switch box. Either track one of those down or buy this, plug it into your service outlet and splice the yellow wires into your 3-pin harness.

https://na.suzohapp.com/products/bill_validators/42-7057-00

You can also use VFM2 pass through style bill acceptors in the upper slot. None of them can be updated for new $5s but in my experience 99% of bills taken are ones and you don't have to track down a slim bezel or cut your slot. These are the only ones that work in Gottlieb Coin Control and early Whitestar doors too.

Correct. You could also terminate the ends of the yellow wires with female ends and plug them right in if you don’t want to cut the harness off, this would be more reversible if that matters to you.