Q: What would cause the heating fail inside the car vents.) on 2003 Toyota Highlander

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Recently, had the radiator changed due to rust deposits. Then when the heating failed changed the thermostats. Still not heating.
(1) Answer
It is the electrical control for the heater on / inside the dash. The temp control switch connects to a circuit board with 3the tiny wires. Over time, 1or more wires get broken. Toyota will replace the entire unit for $1000 (parts and labor), or if you are handy you can take it out yourself and either solder it or hire someone to solder it. A Stereo Repair shop can remove it for you, then look for an electronics repair shop or ask the stereo shop for a recommendation. It is REALLY easy to fix with a low-power, pencil stylr soldering iron. My guess is less rgan $200, unless you're getting gouged. If you were closer to me, I'd do it for you for $150. Toyota will not "repair" these; only replace. Too much money to be made, I guess.