4Runner blows hot air on drivers side and cold air on passenger side AC is fine for both

I purchased a 2005 4Runner SR5 Sport Edition V6 from a friend who said it sat for 2 years in the yard he parks his roll off truck in. He stated all that it needed was a new water pump and he also replaced the thermostat. I took it from OKC to KC and about 75 miles into the drive it started to over heat so I filled it with water as it was in the middle of the night and nothing open on highway. It continued to over heat periodically throughout the drive so I would periodically stop to keep from over heating and possibly causing a blown head gasket. Once in KC I filled with antifreeze and that seemed to stop the over heating problem. The heater worked fine on both sides that first trip then after the antifreeze fill up it stopped working on the passenger side. I seem to keep loosing coolant and don't know why. The overfill is constantly full. I see nothing dripping so I'm baffled. I took the glove box apart and found one of the plastic gizmos near the climate control module wasn't attached so I got it back in its proper place and thought that might fix the problem. NO dice. Now I'm getting less heat than before. I don't want to pull the modules out if that's not the problem I had a mechanic the family has used for years suggest that it sounded like a blown head gasket when I mentioned that a friend had suggested I check the oil for foam and found nothing he changed to something else. Im not really fond of him as he is not his father that he has taken the business from. I mentioned the heater core and he said if there wasn't coolant in the floor board it most likely wasn't that. Keep in mind I have full AC on both sides. I'm baffled and would love some options. Thanks