Sidelights and tail lights not working, new bulbs tried but 7.5amp keeps blowing

[AL B]

Side lights, tail lights and number plate lights not working.

tail light bulbs had blown so replaced them.

7.5amp fuse replaced but blows when lights turned on

dash lights working fine 

any clues?

[IanML]

It sounds like a short to earth on either the wiring or one of the bulbholders.  A rare alternative is a short within one of the bulbs - check this by removing all the bulbs and replacing them one-by-one, checking the fuse each time.  If the fuse blows before any have been replaced, it's the wiring, otherwise it's the bulb you just replaced.

You can confirm a wiring/bulbholder short by removing the fuse when the bulbs are removed, and measuring the resistance between any of the positive bulbholder terminals and earth.  Test with the light switch on and off.  If the short vanishes with the switch off, the short is located between the fuseholder and the switch (could be in the switch itself).  If it doesn't vanish, you have to break the circuit down branch-by-branch to locate the offending point.