You can try different solvents, but it sounds like you've already tried some. Something like Goof Off or Goo Gone might get it. You can also try a heat gun, get it hot and scrape it off with a hard plastic putty knife. If you can get a bit of it and put it in the solvent, you'll see how well it works on it.
You mentioned you tried bug and tar remover, I'd try soaking it. Take a tooth brush or something like it and a dish of the bug and tar remove and keep working it on there. Keep it wet for a while and see if it's starting to do anything. When you keep brushing it on, it's better than soaking because you are brushing the surface and it can dig in faster.
Maybe warm it up with a heat gun 1st, then tooth brush and solvent. One of those stiff nylon brushes should help.