You may have a faulty park switch in the wiper motor assembly. Get it diagnosed by a Pontiac specialist.
If you want to find a GM specialist, here's our directory link for you:
https://repairpal.com/pontiac-repair-in-rock-island-illinois
RepairPal Certified shops can diagnose and repair your 2003 Pontiac Montana. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.