Have you ever heard of X10 products? They run over the house wiring. Not wireless. Only requirement is the buildings are on the same electric feed.
- Is the garage electric fed from the house or the house fed from the garage?
If yes, you can you can use an X10 heavy duty 20amp appliance module for $34.99.
Pam 4 Heavy Duty 2 -Amp Appliance Module - Modules - X1 PRO
Or an X10 heavy duty 20amp appliance receptacle for $29.99.
XPR2-W White Heavy Duty Appliance Receptacle - Modules - X1 PRO
You'll need a way to turn it off and on, so you can do it manually. Here's a controller for $32.99.
PMC 1 Mini Controller - Controllers - X1 PRO
Or put a timer on it. You can still do it manually from the timer. $59.99
XPMT4 LCD 64-Event Mini Timer - Controllers - X1 PRO
I've used these things for well over 25 years. I've used that exact heavy duty 20amp outlet on my swimming pool pump for 10-15 years, plus numerous lights and things on different modules.
The beauty of these things is they are simple, plug in, and work over the wiring.
If you want to get fancy, you can add wireless interfaces for remote controls, computer interfaces for advanced programming, etc.... but for simple plug-and-play of 110V devices, they work.
The company was great, then some sleaseball bought it and turned it into a wireless camera voyeur type creeper guy hangout, then went bankrupt. Someone else has purchased the company, and it appears to be people that are returning it to what it's good at; simple control and automation.
If, for some reason, the signal won't go through, there's a passive coupler you can put next to the breaker box that bridges both legs in the box so the signal goes through, but I'd try it without the coupler first.
Anyhow, it's a very good solution that works without wireless anything.
You could put an LED light fixture in tandem with the block heater/slide pump in a location where you could see if from the house, to act as an indicator light as well, to let you know the signal got to the module to turn the block heater on, and also to let you know when it's off.
Worth a look.