If they are anything like my 97 Grand Prix (which they probably are) and the splines on the drive axles that the wiper arms attach to are not stripped (probably not) I say its most likely going to be the shear pin on the drive cam attached to the wiper motor. Its there to shear off when the wipers are too obstructed, avoiding damage to the motor and other components. I actually fabricated another pin or found an equivilent at the shop I was working at, at the time.
BUT... make sure you make a note on how the cam is attached in relation to the motor and wiper arms at park.
Before disassembling, make sure the motor has gone to park (turning the switch on then off before disconnecting the battery to work... if thats what you do). Then when everything is uncovered move the wipers to park and be sure that the two plates of the cam are aligned at the shear pin. Make sure the cam and arms go back together in this same fashion or else you'll have really weird wipers. When I reassembled mine I must have been 180 deg out or something. And like a moron I totally reassambled everything (was in a rush) before testing them. I ended up with wipers that parked correctly, but they cycled 4" too short and stopped in the middle of the windshield between each cycle lol.
Well now you are prepared for a shear pin failure
(When I say cam, don't think camshaft. Its merely two plates sandwiched and fastened by a shear pin, with a center pivot and an eccentric pivot, the shape of a camshaft lobe profile