[Mgs] MGA 1600 rear light intermittent short

[Robert J. Guinness]

The passenger rear running light intermittently goes on by itself on my 
MGA 1600.  This is the lower bulb that is also used as a brake light.   
It happens most often after I have turned my back on the car and closed 
the garage door, only to come back later to see it on.  Any advice on 
what and where the likely culprits are, and how to diagnosis this 
short.  Thank you.
-- 
Robert Guinness