I have a 2004 Pontiac Grand Prix. The wipers only work on high speed and do not settle when they are turned off.
The problem is probably with the wiper motor pulse board.
When GM vehicle wipers start acting goofy, the pulse board is the first suspect.
Tester
+1 for @Tester .
I’m wondering if it could be a relay, like on a fan motor that will only run on a single speed.