Brake lights don't work right...

I've run into a bit of a problem with my taillights... Everytime it would rain, my taillights would fill up with water and then the bulbs would burn out or explode. So, I finally took them out and drilled little drain holes in the bottom of them and replaced the bulbs. I turned on my lights and checked them and all my lights and turn signals lit up properly. But... When I press my brakes, the passenger side taillight goes black and all the rest work fine. I let off the brake and the taillight comes back on. I checked the bulbs and the filiments are fine... So, I'm wondering what the hell is going on with it. Could previous water have shorted out a wire somewhere in the taillight system or is it something else?

could be a short. How was water getting into the taillights? I would have looked into that first before drilling drain holes.

It appeared to be coming in around the seals between the plastic housing and the actual taillight reflector part... But, I thought they were permanently attached to each other with a seal between them. I donno... Anyways... I also wondered if it could be a short. If it is a short, what would be the simplest way to find where its shorted out at?