Just saw your post while trying to troubleshoot ours. If you haven't already found one of these- check Inland RV for the cover. Our 69/70 Overlander has two wires coming out-one hot and one ground. The fixture itself is a simple socket that you can buy at a car parts store, bulb can be just about any single bayonet type-the manual specks out a #90 but I've been testing our with an 1141 from our ceiling light. The original fixture had a frosted lens cover-if you end up getting one with a cover you might want to an LED bulb to avoid the hassle of changing out the bulb. The manual also shows the bulb and socket mounted vertically oriented above the door. Seems like they were thinking it would be easier to change the bulb if it were pointed down towards the door. The design might intend to remove the lens to change the bulb. Ours did not have a lens so we made a new one out of frosted acrylic.
WBCCI-6849 . TAC OH-22
It is impossible to achieve the aim without suffering-unknown source from a Robert Fripp album-