This has been a common failure on this model. Such a problem that gm has released a bulletin about this problem. The failure lies within the wiper motor control module. This module controls all of the speeds of the motor. When the module fails it causes for the wipers to run all the time. The gm bulletin number is XXXXX You may want to contact your local gm dealer and get a new module pt #88958136. This should cure your poblem. Feel free to contact me with any questions! Good luck! Oh, here is what you will need to do to access the module.
Tighten Tighten the screws to 3 N·m (26 lb in).