Has nothing to do with the underdash harness as far as I know on a 65 - but will result in a different power window harness. There are actually 4 different harnesses - power windows, power windows and power vent windows, power windows and power locks, and power windows, power vent windows and locks.
Regardless, if you have power windows already, then 90% of the battle is won. All you need to do is push the wires for the power vent circuit through the big rubber bungs that seal the power window harness holes in the cowl. I have already done this on a couple of cars - it takes time to do, but it can be done.
You must remove the existing power window harness from the car to do this right, and to ensure that the modifications explained below are done without damaging the existing wiring. The bungs easily push out of the cowl from the inside. Don't try to push them into the car - push them OUT from under the dash.
The big rubber bungs are hard, and are molded around the existing harness. You have to create a new pathway through the bung above the pathway the existing harness uses, which is dead centre. Use a fairly small drill (1/8th), and drill a hole right through the bung above the existing harness. Go slowly and ensure that you're not going to drill into the wiring that's already there. Once you've got that hole drilled, then go up a size to 3/16ths.
You will have to cut off the connectors to the switch (and motor for the passenger door) and reconnect them after you've pulled them through the rubber bung. If the wires are not already wrapped (it's likely best to unwrap them and make sure that things are OK in there), wrap your new wires with non-adhesive wire wrap (don't use electrical tape except at the ends of the wrap to hold it down), and pull the new wrapped wires through the bung by taping them securly to some mechanic's wire. Reconnect the switch connectors and the motor connector for the passenger side. New connectors for the motors are available on eBay - they're kinda fragile and can fall apart. Seal the new wires into the new hole in the bung with liquid rubber electrical insulation - Gardner-Bender sells it.
Take the power for the vent window harness from the same circuit breaker that the rest of the power window harness gets it from - found in the driver side kick panel. You can take it from the fuse panel as noted on the diagram, but it's best to take it from the circuit breaker in the kick panel.Don't forget to disconnect the battery whenever you're working with that stuff, as the harness is always live even with the key off.
I have attached a wiring diagram. Note that the bungs are not marked in the diagram, but that anything that has to come OUT or INTO the doors HAS to go through the bungs in the cowl. Note also that this wiring diagram is different than the wiring diagrams you will see for later 69 and up cars as noted in the thread by @Big_John
P.M. me if you've any questions. The diagram is also attached as a PDF - hopefully you can download that from the page. If not, send me a P.M. and I'll send it to you.