I have a 1999 F-150. I have to pull the fuses to turn off the exterior lights. This just started after the heater core was replaced. Does anyone know what might be causing this?

 

Just reading your thread on the exterior lights staying on after replacing the heater core. I think when replacing the heater core in the 99' is similar to a 98' where the entire dash, air bags, part of the steering column, etc needs to be removed. It seems that they grounded out the wires for the headlights and parking lights when they put everything back together.
Did you do the repair or a shop? Have the shop fix the problem.
My heater core was replaced, and it took the shop 10 days to do it, and said they would never do another one again. PITA!!! I even started sending him Xmas cards after the ordeal.

 

I just did my heater core a few weeks ago in my 99.
Make sure your headlight switch was installed correctly.

Makes me feel pretty good that I pulled it off in under 10 hours let alone days..
Shame on them.
Ford quoted me at 8-10 hours labor plus parts, $1400 or something like that.. I was like eh, $100 in parts, I'll do it myself.

 

Do you have auto lights?