When you fitted the foglight bulb, did you inspect the lugs that hold the prongs of the bulb on the cluster? and the springy tab thing at the back?
( the bit the round effort at the bottom of the bulb sits on?) excuse the technical talk
If a hamfisted attempt to force in a bulb in the past had damaged the contacts in the bulbholder, its just possible that the + and - 's are touching, causing a short?
Its begining to sound like an auto-electricians job is this
I cant help thinking that the wiring has been bodged somehow,when the original reverse was converted to fog?
Check the colour of the wiring at the back of the foglight switch for clues to the colour and guage of cable used for the conversion,then check the wiring to the rear light cluster for the same colour wires,to look for a fray, or peeling of insulation etc... good luck
