On Tuesday, October 22, 2019 at 8:51:59 PM UTC+1, wrote:
wrote on 22/10/2019 :
I'd start by removing the lamps so there's zero load, then replacing the fuse
with a resistor, and monitoring the voltage on its output. Disconnecting bits
where accessible will show you when the short goes away.

A small 12v lamp would be even easier to use, instead of a resistor.
Lit would suggest the fault is still there, unlit that there is no
short.

Thanks to all for all the suggestions. I'll have a root around on Saturday. I did get tail ended twice last year.One was just a new bumper the other was severe enough to set off the protection system. Seat belts tightened, head restraint snapped vertical (triggered by a small explosive charge apparently). That may have some bearing though all the damage was repaired last year