Ok, I am posting because in my searching I haven't found someone with the same issues.

Car: Completely Stock 2005 Scion tC. Stock Head unit, stock everything.

Issue: When Headlights are turned on, Radio/HVAC AND Dash Lights will not turn on.

Troubleshooting so far:

The dimmer switch is turned on, not off
Does not appear to have any loose wires in the cabin or under hood.
The fuses are all fine.

I have come across multiple threads pointing to the common 2007 HVAC issues, but the additional Dash Light issues do not seem to fit that issue very well. So I wanted to just shoot this question to the forum, and see if there are any other known issues that I should start with before I take apart the dash and try the HVAC fix.

 

Might as well just try it bro. Mine did the same thing but i have an 07 so the HVAC solder fix worked for me. No harm in making sure they are on right! It literally took me less than 30 minutes to disassemble, solder, test then reassemble.

 

That's the plan if no one else has any ideas. I may try and replace the dimmer switch first