On the 97 Blazer, Bravada, S10, etc there was a well documented problem of a cold solder joint on the control board. Not sure if your 98 Buick might have the some control board. The fix was pretty easy, pull the control board and apply new solder to the weak solder joint.
When it happened on by 97 Bravada, it showed the same problem you are experiencing. Wipers would work when dry, fail when raining.