You definitely need the voltage stepped down, and an ignition ballast resistor does not work, it reduces the amperage too much. A 6 volt wiper motor on 12 volts really moves fast, and the can live a while if you don't run them for long periods of time. I think the 6 volt one on my 54 Dodge lasted a couple of years until I was in a pretty heavy rain one day and smoke rolled out from under the dash as the wipers stopped at mid stroke. It was a pretty exciting few minutes shortly after that, had to pull over to open the windows and let the smoke out of the cab.

I think I would look into a remote electric lock system, the original stuff was pretty lame. There is only a lock on one door, the passenger side here in the states, and it looks like it was an after thought, a very strange setup only used a few years. I know there was a set screw that held the key lock cylinder in the door, and a square rod that is attached to the back of the key lock cylinder goes into the latch assembly. If you push the inside door handle forward, it will lock the door from the inside. If your key lock assembly is still intact, and you can get it out of the door, a locksmith should be able to make a key to match your lock tumblers, or change the tumblers to match a key you may have. Most I've seen, the key cylinder self destructs upon removal from the door.
Gene