Help with a lighting problem

Hi All,
Voyager 1997 (MY) 2.0L LE
UK spec

I have an odd problem with the side lights on my car.
they are very dim except one.
If I remove one of the bulbs the rest at the back go out.
Initialy I thought bad connector or bad earth but having looked
at the rear light clusters the tracks suggest that the lights are in
some sort of series configuration (i.e. the contact on the top bulb
that would be negative,outter case, goes to one of the positive
contacts ont the stop/tale bulb????
alsowhen I look at the wiering diag on the CD I bought it shows
the light switch to be switching neutal????
Any help or thoughts would be most apreciated.

Regards

John