I own a 7th gen with 300,000 miles. About a year ago had a bad radiator which was replaced. But now when you turn heater on the car overheats still, but not when you have the A/C on and it’s 100 degrees outside. I’ve turned the A/C on and left it running for over an hour in the summer heat and it never overheats, just when you turn on the heater, I checked radiator fan and switch they both work and operate normally. A/C fan functions too. My passenger side floor board was wet but was determined as a bad door seal by mechanic. I’ve had him test for a blown head gasket and it all read 180 on all 4 so he said it was ok. Is it a clogged heater core or???? I’m not sure where to start.