It sounds like the battery drain issue isnt your central focus at the moment, but I'll comment on it anyway.
While I haven't dealt with electrical issues on the S-chassis specifically, I have had quite a bit of electrical experience on other platforms. If your short is in a body/engine wiring harness somewhere, it can be VERY difficult to trace. The best place to start is the fuse box. As somebody mentioned earlier, with multi-meter in hand, start pulling fuses until the drain disappears. That helps you to narrow it down at least.
Another place to look is the alternator. I know it sounds stupid, but I have had more than one car with a battery drain that ends up being a bad alt. The car would still charge when running, but there must have been some kind of a short to the body of the alt (which is the ground) thus creating an open circuit and draining the battery. I would recommend testing this by removing the power wire to the alternator (and wrapping it with something so it doesn't short out on anything) and letting it sit for a few days. If you come back out and the battery is still dead, then I would assume your short is elsewhere.