Sounds like it could be a heater core leak. The two pieces of trim covering the HVAC unit, which is all located on the front passenger area, are fairly easy to remove. I'd first check the clamps in there.
Does your front passenger windshield fog up?
1998 TD 4 Door Hard Top (HMC4) w/ 89K miles
Updated block and loaded with parts from Hummer Parts Guy and Blue Hummer
HummerDan's Hardcore Skid Guards, Fender Guards, & Locking Fuel Doors; Ham (VHF/UHF) & CB radios; Predator RapTorq4 PCM; mechanical wastegate; Cepek Adjustable Shocks; M101A3 trailer.
Stanadyne Performance Formula every fill-up and ScanGauge II full time.