Ken, I have an '88 Westy and also wanted an interior light over the
passenger door.  What I found is that the light over the driver's door has
a metal plate behind the headliner that insures that the light 'fixture'
has something solid to latch into.

As I recall, the passenger side of the headliner on the Westy had no hole
at all for a light on that side.  My solution was to get one of the metal
plates from a 'donor vehicle', cut the hole on that side and add the
backing plate to make it just like the driver's side, and of course, run
the wiring over to that side above the headliner.  It's a nice addition, I
think

John