I purchased a regular 12V red taillight, took out the 12V bulb and put in a 6V bulb. At first it looked just fine. After maybe 10 minutes the ammeter needle shot off to the side and the wire on the tail light melted off. Was the wire just too close to a ground or do you have to have heavier wires for 6V lighting? I noticed that the wire from the harness was much heavier than the wire from the light.
Maybe I should just put the kerosene lamp back on the tail and carry a book of matches around with me.
I did it and had no problems for the past 5 years. The light isn't pulling that many amps. You apparently have a short to ground at the pig tail. Pull the pig tail out of the socket and I think you'll see the obvious short.
Dave, you are right that a 6V bulb with same wattage as a 12V bulb will pull twice the amps. However - for a taillamp you normally just use 5W which will only draw approx. 1 Amp. Normally what we over here call a 0.75 mm2 can cary that as Kenny also indicates.
The fact that the Apm.meter shot off indicates that Kennys theory of a short somewhere is right.