Hi Jerry,
It's Scott. Sorry to be out of touch. What year is the new truck? Below is a wiring diagram for the 91. It should be the same as an 89 or any of the B2600i trucks.
When you said you replaced the switch, did you replace the entire column switch assembly or just the wiper switch portion. Also, does it have intermittent wipers, or just the two speed (slow / fast)?
If it has intermittent, there are some electronics in the switch that could be faulty as well.
Was it draining the battery before you replaced the switch? Or only afterwards? Do the headlights (high beam/lowbeam work OK? How about the turn signals and hazard lights?
The reason I ask is because the headlights (high beam) and hazard lights have un-switched battery power passing through the switch. So if you have some kind of mild short in there somewhere, one of those switches on the larger combination switch could be causing the problem.
If you think that one of those switches on the column is at fault, then I would unplug all of them. Take note of the battery voltage, and then plug each one in, one at a time, and watch for any dip in the battery voltage. If each switch is "OFF," then you shouldn't have any draw on the battery.
Also, you might want to check out this thread for cleaning the switches:
Combination Switch Exposed - " target="_blank"
The wiper switch is much like the headlight switches. Here is a link with detailed pics and operation of the wiper switch
(you can scroll down to post #15.)
" target="_blank" target="_blank" target="_blank" target="_blank
If the image is too small to read, you can view it full size here:
" target="_blank" target="_blank