Rear light issues

[offrink]

My ‘54 fcub has a bulb type rear light. It was dim but ran when the lights were on for years and then it stopped. I ended up using a new wire from the front light switch relay (yes I know it’s wrong) to the rear light. I know it has power running to it using a multimeter. I took a jumper wire and grounded the frame of the light to the battery (positive) and the light brightly light up. I ended up taking that grounding wire and ground it to the frame and when I turned the lights on the fuse blew. I didn’t think these lights needed to be grounded but it seems it does. So what did I do wrong?

[inairam]

Have you had the light apart? for a 54 - Is this a pancake light? TM has replacement parts for pancake lights but you have to call. It is not on their web site. If it is a teardrop light they also have parts but those are on the website.

I have taken a lot of both styles apart and the conditions inside are all over the place. Some are in very good shape some are a mess. You probably have something shorting out.

[k hutchins]

The first place l would check is the "hot" wire. Vibration over the years can cause bare spots in the insulation. Which then short out. If that is fine, next would be inside as lnairam says.

[offrink]

What would I be looking for inside the light? It is just a bulb with a spring attached somehow to the power, flat plastic disc and a peg. Electricity is metered between the positive on the battery and the metal on the bulb and reads 6? Volts without a ground wire.

[Stanton]

You’d be looking for anything that would cause a short, like worn insulation.
While you have the light disassembled, use some fine sandpaper on the soldered nub at the end of the pigtail, the part that makes contact with the base of the bulb. Also sand the other end of the pigtail where it makes contact with the terminal screw. Sand the brass tab that makes contact to the reflector, that is your ground.
Double check the condition of your light post and bracket, which also is your ground.
When you said your light brightened when you ran a dedicated ground, that would indicate a connection issue.