Hi Graham, i have done one of these recently, if you take the panel off that holds the light switch its in there i think it's 440 or something near there are 2 relays with the same number and it was the top right hand one it's in a relay holder by itself
The fault with my one was the earth dropping out which is on the inside of the nearside( L/Hand )rear panel mine was a fridge van so hence it was lined took me bloody ages to find also the near rear light had a fault.