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Fighting a pretty aggravating problem. I have an 09 tC, and the HVAC and Radio Lights go off when the headlights are turned on. I have read a lot, and tried the typical things. I have the advantage of having 2 of these cars, so I can troubleshoot a little more effectively than usual.
I have swapped the Radio and HVAC Controllers for one car to the other. All hardware works perfectly normal (dims when headlights are turned on) in the Black car, but the problem remains in the White car while using the parts that worked fine in the Black car.
This seems like it is an electrical issue which is tricky and very time-consuming trying to locate a possible short/issue. Going by what you said it could be a short in the dimmer/illumination circuit, but I could be totally wrong. I would check for blown fuses first for any possible shorts. Have you tried unplugging the stereo and leaving the HVAC plugged in or leaving the radio plugged in with the HVAC Unplugged? Also, is your stereo stock or aftermarket (I don't think it makes a difference but it might).
I have a 2007 Scion tC wiring diagram manual PDF, this is a color image that I found compared to my black and white manual
This might help if you want to look through the online manual.
Yes, the problem is definitely in the White car. All components are stock. And if I leave the HVAC unit unplugged, the radio lights works like they should (on with switch and slightly dimmed with headlights on). This made me feel like the problem was definitely the HVAC unit.
So at this point, I swapped the HVAC units between the two cars. Then, the unit that had been working in the Black car did not work in the White car, and the unit that had not been working properly in the White car worked fine in the Black car.
Update. With everything connected, the lights work with the headlights off, and (instead of dimming), the radio and HVAC lights go off completely when the headlights are turned on. With either stock radio installed, and no HVAC control module installed, the radio lights work as they should (on with no headlights, dimmed with headlights on). With either stock HVAC control module connected, and no radio connected, the HVAC control module lights work with no headlights, but go off completely when the headlights are turned on. As I said previously, I have two tC’s, all of these components function normally in one of them, so I know the modules work properly.
HVAC/ Radio Wiring Diagram
