dean Hi, the secret to electrical fault finding is splitting the ccts if possible, difficult with that cct, you know that with the lights turned on starting the engine causes the fuse to blow, so , as jk said, can you disconnect all the lights at the bulb holders, then turn the light switch to on and start the engine if fuse doesn't blow then problem would seem to be at a holder, if fuse does blow then its the wiring somewhere, more of a problem to find as its pretty much only physical inspection that will help, its a big step but you could cut wires to split cct and use Multimeter to find shorts, then use in line splice to rejoin wire Tim