The base of where the nut threads on snapped...

 

Is there a way to fix or do I have to buy a whole new taillight assembly (if I can even find one).

 

You may be able to drill it out and fit a new bolt and nut.

But be careful removing the other nuts when you remove the unit to work on it, it looks like you've got a good deal of that Michigan corrosion going on.

Used fixtures can be found, check ebay or the parts forum. Depending on the condition of the rest of the unit, you may want to consider this option.

 

The whole unit is perfect and no rust I just accidentally tightened one of the bolts where the nut goes on to much, not a problem with anything else on it I've took it out several times with ease. The black stuff is this real thick grease/ tar I have no idea. But thanks for the advice nobody likes it when this kinda stuff happens lol

 

Yeah, it's a bitch.

Does the light seat nicely against the body without that nut?

If so, you can probably just ignore it.

I like problems that fix themselves.

 

I guess it's fine. I'ts not like it's loose or anything because all the other nuts are seated and tight. It's just knowing it's not 100% right you know what I mean? haha