Per trader_4:
IMO, it's either fix it yourself, which you might be able to do if it's
just the defrost, or get a new one.

Just ordered a replacement for the obviously-funky defrost heater.
Replacement includes thermostat.

I'll put those in, expect to get at least a couple weeks out of it even if
that's not the problem, and start pricing replacements.

Replacements are a little tricky in that it is a smaller unit and the space
it has to fit in just barely accommodates it.
--
Pete Cresswell