LH side light fuse keeps blowing.

OK, getting frustrated now! The fuse for the LH side lights keeps randomly blowing, obviously there is a dodgy connection somewhere, but I have tested the lines with a circuit tester but whenever I test it it's fine. I have also inspected the wiring as far as possible without taking the whole thing apart, question is, is there anywhere in particular that is susceptable to dodgy connections or chaffing etc?
Any help much appreciated.

I think the section of wiring over the front wheel arch and down to the lights can be problematic, if it's not blowing straight away but working for a while, it sounds like a wire is rubbing somewhere.