Electrical Insulation in Built-in Barbeque
What is the best kind of insulator to use


What does the manufacturer say ...



I suspect the roof seal doesn't get too hot in normal use. You can get "heat-resistant silicone" intended for use with flues etc.

Personally, I'd get the metalwork properly earthed (grounded) to help avoid killing family or guests.




  the problem is moisture getting in-between the steel casing and the brickwork, flowing down the wire


That is possible, inspection of the fitting may make this clear


  I don't really see another way.


As Ed Beal pointed out, soot is largely carbon and carbon is conductive. Maybe soot is getting somewhere it shouldn't.

I would examine the light fitting and its connections very carefully to try to identify what is creating a conductive path from the hot (live) wire to the metal frame.