I have to replace a thermostst. How much is it? (part&labor)

Hello, This may not be a bad thermostat, in fact, it may not be a cooling problem at all. Run your estimate at the link below for coolant temperature sensor replacement, and general diagnostics. This one should not take very long at all to figure out, especially if you use a certified shop with highly trained personnel. If you need further assistance, let me know. https://repairpal.com/estimator/volkswagen/passat

Sounds like a coolant temp switch issue especially if you have good heat. These are generally inexpensive.