There are ways to apply more torque without distorting like the vise grip will. But being such soft material it's likely to gall and tear the threads off the housing. Also using some heat could increase the clearances but again probably not much chance of success with soft metals.
I expect the combination of electric current and moisture has corroded the threads together.
I'd try to get a replacement cap and cut the old one apart because you will likely be able to save the housing.
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