For the second time in a week my side and tail lights have failed soon after I switched them on. This is a 10A fused circuit that powers the two front side lights, tail lights and rear reg plate light as well as dash board instrumentation. The fuse is blowing.

I had also added illuminated running lights on this same circuit so immediately suspected this wiring. I removed all of it this morning but still only had 1.0 Ohms resistance between 12V and gnd consequently a 10A fuse would just blow again. I next removed both side light bulbs, both tail light plug connectors and the reg plate plug from loom ie so it just leaves (I think) the dash lighting on that circuit.

I still have 1.0 Ohms resistance between 12V and gnd so what do I do now ?? It's beginning to look like I must have a dashboard wiring fault. Problem is really no tail/dash lights for driving home in the dark. Is it auto-electrician time yet ?? (1/01)
I'm afraid it looks like a chafed wire in the loom. You'll probably save yourself a lot of time and aggravation if you consult an auto-electrician, much as I hate to say it. Just removing/replacing bulbs doesn't isolate the wires that drive them - you need to disconnect plugs or cut wires. It's quite possibly in the boot - you could try unwrapping the loom where you can get to it. (1/01)
Have you got a dimmer switch fitted to your car, or have you had any of the blanking plugs out by the rear fog switch, or is there any other work been carried out lately other than your marker lights e.g. alarm, new stereo ? (1/01)

I have traced the problem....one of the bulb holders from my added running lights had corroded and there was virtually a short circuit to ground so when I turned the sidelights on, the fuse blew.

I remain a little mystified by the sidelight circuit resistance measured (obviously) with power off. Even after removing my running lights, it is around 0.8 Ohms which I would have thought would mean I would pull 15A when the lights were on. This is clearly not the case as I proved my fix with a 5A fuse and it didn't blow.

Perhaps someone with a multimeter would be good enough to measure this resistance on their car for comparision. An easy way to do it is to unplug the rear no. plate connector and measure "into the loom" if you know what I mean. (1/01)

1.8 ohms - measuring at a rear tail light bulb holder, so without that bulb but with my four lit side markers at 4W each, so a bit less resistance than standard.

1.8 ohms pulls 6.67A @12v = 80W 1 tail @5W + 2 front @5W + 2 No plate @5W + 4? dash lights @4W + 4 side markers @4W = 57W Right sort of ball park - bulb wattages are pretty approximate, and this is a fairly crude measurement with all that wire involved.

Yours does seem to measure low - did you remove the "tail" fuse to see if that has any effect - it may be feeding the alarm or something and that may affect things. (Hard to see, I know.....) (1/01)