Vapor barrier behind electrical panel?
Since the garage is not within the conditioned envelope, then no

The key to understanding a "vapor barrier" (really, an air barrier) is to understand that it is part of the system that separates "inside" aka conditioned space from "outside" aka unconditioned/outdoor space.  So, your air barrier needs to go with the other barrier layers, in particular your thermal barrier (insulation).

From this, we can determine that since your garage is unconditioned space, the air barrier should sit on the ceiling along with the insulation there, instead of being on the wall behind the panel, divorced from the insulation and letting unconditioned air get through the insulation as a result.