1996 Chevy Tahoe LT. Whenever it rains hard, I get water leaking inside through the heating system. It drips out a same hole in the bottom of the fan housing. Where is the outside air intake for the ventilation system? Could this be where the water is getting in. Car does not have to be in motion for it to leak.

Your climate system can only cool or heat but not both. The temperature has a big effect on this. The cooling needs the compressor.

To repair properly, should have w/s removed and necc repairs performed

You must replace it, that is a heck of a job and not for a person with no auto repair sjills.

i believe you have a leaking heater core, if the water is leaking out from under the dash right under the glove compartment this is almost certainly the problem

A leaking windscreen,door seal or window seal. Run a garden hose over your car and use baby powder to pin point where the water is in running from.

a 3/8th hole will suffice so long as you remember to keep it clear,as for the build of these vehicles i have no idea ,sorry i live in spain so ask me about a seat or citreon and i recite it from memory

Water goes between the front and rear window and is supposed to go through a trough and drain out of the bottom of the car. often the drains along the bottom (4 very small slits) get clogged and water will back up into the floorboards. just drive a flat head screwdriver in these slits and it will let the water out! (watch out about 2 gallons came out of mine!)

That "thing" is part of the compressor clutch. When it’s spinning the clutch is engaged, when it stops the compressor is off. If it’s constantly cycling, you probably don’t have enough freon in the system. But remember, however the gas escaped before, it will escape again, unless you fix the leak.

It sounds like your winsheild gasket has a leak. There is a small rubber gasket that goes around the windshield. If it is leaking air, it may wind up leaking water, rain...etc. There not cheap. It would cost around 100 or so to replace.