I give up...

I assume you've tried a different bulb?

Has the pigtail / socket been replaced?

Don't remember on the pickups----are they an all plastic lamp, or in any case do they have a separate ground wire?

It has to be one of three things.........

Bulb, ground or power

Maybe the way you are checking is throwing you off.

Let's say there's a break in a harness plug, l just enough to light the test lamp. Maybe when you install the bulb, there's not enough "connection" (high resistance) to carry any current.

How are you checking for ground? With bulb in, power on, stick your meter or test lamp onto the socket metal, and to a good body ground. You should read no voltage/ the lamp should not light.