Thanks, I wasn't sure what it looked like and that helped me out.
I fixed it. Testing at the module side I was getting nothing from the sensor. So if anyone finds this later, here's some tips.
The sensor is not a photodiode, it's just a regular CdS photocell in a cute blue plastic holder. The big black dome thing is the sun load sensor for the A/C, the headlight one is in the little hole behind it. I never noticed that before, and I thought the domed thing was the headlight sensor.
And that's the key in this case, the sensor is down at the bottom of a hole. The defroster grill was really cruddy, that crud fell in the hole, and the sensor was totally blocked by same crud.
If your lights stay on all the time, try cleaning out that hole with cotton swabs or something similar and see if that helps. If not the defroster grill can be pulled straight up to access the sensor from below. Use caution as the plastic there is exposed to sunlight and can get especially brittle.