I see TONS of threads on fixing the 07, but my 05 has the same symptoms:
HVAC/Stereo are lit with headlights off, with headlights on they go dark.
Ive only found a couple threads concerning the 05, but they all do dry without a resoultion posted. I removed the HVAC control board and the resistor thats in the 07 is not to be found.
If it helps I bought the car used and know that once upon a time it had an aftermarket deck in it.

 

swapped HVAC controls, didnt fix issue.

 

after some expermenting I unplugged the HVAC and left the stero plugged in. In that configuration the stereo lights up normally (with headlights on) as soon as I make the wire connetion to the HVAC the stereo light goes out...something to do with the HVAC wiring...I need ideas!

 

Long ago a friend of mine had the same symptoms with his HVAC control assembly as you described. Being the good samaritan, I let him borrow mine for a couple of hours and it ended up operating as designed with my HVAC unit in his car. Not sure if you have the same internal issues with yours as my buddy's, but I'd look into junkyards and find a HVAC control unit for cheap. They are expensive from the dealer.

 

I've been using 2 diffrent hvac controllers, found one on ebay for dirt. outside chance both have the same problem but very unlikely.

 

That's why I don't buy a lot of things from ebay. Chances are is that you bought someone else's defective unit.

 

true about that. my unit came out of a totaled car. It wasnt just something someone was trying to get rid of. I got 30 day return etc on it.

 

I would not recommend doing this but if you have a friend with a tC that you know his or her HVAC is operating as designed, ask to swap units for a couple of hours for some colateral.

Again, you may have a different issue my friend had.

 

I think your HVAC wiring that plugs into the back is going bad. I've gone through 3 hvac controllers ( all from the toyota dealership under warranty). They work for a few months, then go out. Got the wiring replaced & everything has been fine since then.
*knock on wood*