[MkVer]

I run off the grid for my car and truck though I've toyed the idea of a battery with solar trickle while I'm at work and then grid charge at home.

Car. I have a Frostheater/zerostart coolant heater. It works well and I'm very happy with it. It also works for the transmission but that's because my car uses a oil/coolant heat exchange to cool or heat the transmission fluid.

Truck. Block heater for the engine and a pad heater on the transmission pan. There's a huge difference in the transmission's behavior with cold or warm fluid. I like the idea of a plug heater but I worry about fire. Unfounded, I know plug heaters are safe but my subconscious can't get past it.

In both cases, I've found a grille block and hood insulation work amazingly well at keeping the engine warm while the vehicle sits in the parking lot at work. I'm not sure if a heater on the diff would be worth it, that's a lot of metal exposed to the air and frictional loses due to cold fluid I think would be minimal compared to the energy required to heat it.