Dome Light: How it works and Troubleshooting
This thread explains how dome light works, and IF it does not work, what could make it NOT work.
SCHEMATIC OF DOME LIGHT (Verified on rad4runner's 1986 22r-E dlx)

When good battery is connected there is steady 12V at dome fuse and at positive side of dome light.



DOME SWITCH IS SET TO DOOR CONTROL:
WHEN LEFT-SIDE DOOR IS OPENED HERE'S HOW A PROPERLY-FUNCTIONING DOME LIGHT WORKS:


WHEN RIGHT-SIDE DOOR IS OPENED HERE'S HOW A PROPERLY-FUNCTIONING DOME LIGHT WORKS:


Any interruption to circuit will cause dome not to work. We will address various scenarios as they come.

TRUCKS WITH "ILLUMINATION PACKAGE";
Based on discussion on Charles4x4's thread, I believe the diode pointing up and down and in blue rectangle below is inside the "Key in Ignition" buzzer module. The straight wire (X'd out) does not exist. IF the key in "ignition buzzer" is removed, dome light will not work with left door open.

Would be nice if someone could verify by looking / probing their own key in ignition buzzer.

 

My 4R dome light doesn't work, thanks for this guide Ray, I'm going to use it this weekend.