My vw passat heater blows cold air

You may think the heater core is okay, but if someone used a quick-fix stop leak additive on that radiator prior to replacing it, the heater core may be clogged. Also, verify the heater control slider is actually working the bypass valve properly.

Scott is right, be sure to check both coolant lines for the heater core that they are equally hot. Burping all of the air out of the cooling system on your Passat is also a real challenge. Have you run the car through multiple heat/cool cycles?