Leaking water
 

When use the air conditioner in my '91 Silverado, some water drips from beneath the glove compartment.

Anyone know what I need to do to troubleshoot and fix this?

There is a drain tube on the firewall inside the engine bay on the passenger side. Raise up the vehicle and get underneath to find this tube. Its bout 3/8" and will tend to clog up like rain gutters. Stick a wire in there to break up the clog. Some even use air to blow it out. This is very common and you may need to do this at least once a year.

Is it really necessary to raise the truck to get to the drain tube? Where on the firewall is this tube located? I popped the hood and looked around the accumulator but could not see it.

You can only access it from the bottom. If you can fit under without jacking then try it. Its gonna like if its directly behind the glove compartment.

Is it right next to radiator coolant lines that come out of the firewall? There's an opening there (no tube though) and it's the only opening I can see that might fit the description.

Does the opening face the front of the vehicle, or does it open downwards underneath the glove compartment?

Thats probably it. Thats how it is on my 99 Tahoe. I pushed a rod through there.