I have a 2004 E350 and I have an issue where the dome light will turn on intermittantly. I can't narrow down exactly what triggers this, but I have narrowed it down to when I am driving slowly, turning the steering wheel (pretty much fully to the left or right), and/or going up or down a small incline (like a driveway). When it does this, it is accompanied by a small "click" sound. It's almost like one of the door sensors is malfunctioning, but I have checked all of them (to the best of my ability) and they seem to be functioning normally. Does anybody have any suggestions on what might be causing this issue? Thank for any help!

 

Locate C221 remove it for a bench test. How to bench test a relay is beyond the scope of this topic, you can search google on that.




Pinpoint Test B begins here: (you can skip B1 and B2)




If the C221 is found to be bad just replace it but before doing so check and see if you have any unused or less important relay that is identical to C221 so you can borrow for testing purpose. Never forget to also check for corrosion to any of the relevant connector, wire brush them and use contact cleaner and clean them afterward. Courtesy_Lamp_Wiring.pdf attached below.
Also note that almost all door switch use normally-closed type and is energized by its ground signal. That said, when it's short to ground, or have contact to chassis ground during driving, that actually energizes the dome light.
Since you didn't specify the engine size, I randomly picked 5.4L SOHC hope it didn't matter. Check my signature below so next time you post a new topic correctly.