Hey guyz, this might be of useful information for some of you. I finally found the cause for my domelight not working when set in the 'auto' switch position.

What I noticed during this malfunction...
-my domelight didn't work in the 'auto' switch position when I opened my doors or unlocked my doors with the remote
-my domelight still worked in the 'on' switch position.
-all my courtesy lights were still operable on all four doors (which indicates your door triggers are getting 'ground', so they are fine) when the doors were open.
-my factory acura alarm would still trigger if the doors were open, but my aftermarket alarm would not trigger when the doors were opened.

With the help of diagrams provided by avs007 on acurazine and the use of a digital multimeter, I was able to troubleshoot that the passenger multiplex unit was shorted (theres 3 different multiplex units). The circuit that was shorted was the ground output to the domelight. The multiplex unit controls many other electrical things, but in my case it only malfunctioned the operation of the domelight. I replaced my passenger multiplex unit and now the domelight is working flawlessly. If anyone needs a passenger multiplex unit for a 1999 TL, I have a brand new one still sealed. I ordered one online, but it took so long so I went to the dealer to get one. So now I have an extra one. drop me a pm.

*Keep in mind, if your domelight is not working while switched in the 'auto' position. It may be causes other than your passenger multiplex. I believe for mine to be the most expensive. Other possible causes:
-one of your fuse(s) is burnt
-one of your door trigger's is faulty
-domelight unit is faulty
-it may even be a burnt domelight bulb

I want to thank the others on the board that took time to help me out. In turn, hopefully this will be of use to other members on this board. This information should be good for 99-01 TL's. I would assume the 02-03 TL's passenger multiplex unit to operate the same controls. I'm still a noob, rep points would be appreciated.

 

just another point along with your find,
i had a similar problem with the dome light not working on auto
when you take apart the dome light assembly i found corrosion on the metal of slider on-auto-off, just sprayed some rust off and sanded off the corrosion
cured the problem -j-

 

Good find J

 

Mine was the driver's door plunger switch--replaced under warranty