Hi steve1960, welcome to the forum. I suspect that no one responded to you simply because this is probably the first instance of this problem. The rear dome light is fairly simple: power comes in on one pin, the variable 'ground' (from the ETACS-ECU) comes in on another pin (which provides the dimming actuated by the door switch) and there is an internal ground contact to the chassis, most probably through a mounting screw. The switch has three positions: OFF, DOOR, ON.
The front dome light has a switch in the middle which enables or disables the door-switched circuit. Each of the two lights can be turned on by simple pressing the cover. I just discovered that I've probably had that center switch turned off for years...
Are both of your dome lights affected? Do each of the two front dome lights work ok?
The wiring diagram for this circuit is in the manual GR00000500-90.pdf Page 114. There is a link to a manual in our Tech/Reference Section http://myimiev.com/forum/viewforum.php?f=19
, but I've personally not pursued it. Steve1960, if you'd like a copy, send me a PM with your email and I can send you a pdf of this page. Part of my negotiations when buying my first brand-new i-MiEV was that a workshop manual was to be included as part of the deal.
You said you measured power coming in, but did you light up a bulb in order to rule out a high resistance in the power input path? Did you measure between that input point and chassis ground within the dome light itself, or did you put the ground prod elsewhere?
You did get the bulb to light up before installing it?
Presumably you've checked the bulb and its contacts on the light assembly. What happens when you wiggle the bulb or move the switch or do both at the same time? Perhaps do a resistance check between the internal switching points?
These are just some random ideas. After all this, would appreciate a note telling us what you did to fix this. Good luck.