same set up in two of my trucks now. Same problem. I wrapped the switches in the first PPV inside a rubber glove and sealed them up. this would work for a few months to a year. On my new set up i drilled a hole threw the interior sheet metal door skin and pulled the switches inside the truck and made a small metal box. This has lasted two years no issues.
It sucks that we can't just eliminate the rear switches all together
Oh and by the way the switches are going bad because they are now exposed to the wet side of the door were moisture and heat are constant. Nothing to do with the ac being on. in stock form the switches sit inside the vehicle within the dry side of the seals and moisure barriers on the doors