My wagon keeps blowing light bulbs in the break lights. It rotates through all three, the oldest one burning out. I have replaced each of them twice now over the past six months. I seem to only get about a month before the next one burns out.

What in the world is going on?

Measure voltage at the terminals when the brake pedal is pressed and see if it is high.
Are you buying name-brand bulbs? Some are REAL CHEAP and not worth buying.

I'm getting the regular bulbs sold at auto parts stores. One of the brake light sockets has some corrosion on it.. I brushed it off with a wire brush, but it looked like the whitish stuff you see around battery terminals. Hmm, driver side... is the rear wiper fluid reservoir near there?

That gives a clue. Deoxidize the terminal using electronics cleaner or, better, DeOxIt from Caig Labs. Protect all terminals with silicone dielectric paste. Look for the ground plane for the taillamps (trace using the black ground wire: it's inside the pillar, reachable through the hole) and clean that connection as well. And look for a leak in the rear washer tubing.