I, along with countless other GP owners, have experienced this problem first hand. In most cases it is due to the AC Drain Plug being clogged and thus water overflows into the passenger floorboard. The AC Drain Plug in underneath the passenger side black cowl (accessible by going under the hood) and simply unclogging it.
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Rich Ramos - '98 Arctic White GT
Mods: FWI, Custom Exhaust, ZZP Rockers, ZZP Stage 1 HVTB, Buick STBs, 180 T-Stat, Blazer 3200s