You've got a bad blower motor that's going to have to be replaced. It's located in back of the glove box. Do a search, there's a lot of threads on this already including some with write-ups.
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1998 Pontiac Grand Prix GT
WizAired CAI, Impala 12" Brake Upgrade, ZZP Alternator Rewire and Voltage Booster, Rotated Dogbone Mounts, Kicker KS690 6x9s, JBL GTO427 4s, JBL P652S 6.5s
2001 Ford F150 XLT SuperCab
Stock, LineX, 212K