Interior courtesy light problem

[Steve1999Silverado]

I have a 1999 Silverado 1500 LS and my interior courtesy light / dome light does not work properly. I believe the problem is either the door switch or the headlight switch but don't know how to figure it out.  I have squirted wd-40 into the door latch with no change. I do not believe this is the problem beause when I open the door the message center flashes security and it goes out when I close the door.  So the door switch seems to be working. The headlight switch works somewhat normally. With the dome override on (the dome light is off), I can turn the dome light on by rotating the dimmer switch to the top. With the dome override off the dome light stays on and will not go off. Another thing that seems strange is that the procedures I've read to turn off the daytime running lights does not work for me.  Possibly because the headlight switch has a problem.  How do I test the headlight switch?  It's $60 and I don't want to replace it if it is not the problem.