Strange goings on with the rear light fuse...

I just bought a 300TDI 110 and being the first day without rain, I thought I would check it all out and change the bulb that my wife said was out when she followed me home the other night.
Well, its not the bulb, its the fuse, then another fuse and another and another and so on...
After 8 or 9 fuses I have worked out the fault.
Lights on, all OK
Ignition on, lights still ok.
Crank engine with lights on, fuse blows.
Crank engine with lights off, fuse ok
Put lights on with engine running fuse ok.

It seems then, with the lights on, cranking the engine breaks the fuse, this is just the passenger side rear light, all others ok.

One thing I have noticed is that the LH and RH are labelled incorrectly, is this a know discrepancy?

I have looked at the wiring and all is OK at the back of the fuse array and the lights themselves have the wiring connected intact and no signs of shorts.

I checked the wiring diagram and cannot see anything obvious that would contribute to this as the circuit for starting is isolated.
I couldn't find anything about this issue, does anyone have any ideas?
I have run out of fuses now and will have some delivered tomorrow to continue fault finding. BTW, it should be 5 amp fuses and there were 7.5 in there, I did think of putting a bigger fuse in there to see if it would blow, say at 10 or 15 but I don't want to have a meltdown...

Looking forward to some suggestions

Cheers
Bruce

Check all your earths at the lights etc first