That's really useful for when I fit my Lidar jammers. I'd just add that I've always used welding rod or similar diameter stiff wire with a small loop in the end to drag cables through grommets.This guide shows how to and where to pass a cable from the engine bay to the interior if that's what you're looking for:
Read down it until you get to the section that explains running cables through the firewall
There's something about exposing a mains socket to the harsh temperature and moisture extremes of an engine bay which I'd find quite shocking if only I could put my finger on it....Thing I'm wondering here is, if it needs plugging in, what exactly do you want to try and pass through the firewall from the engine bay? You've no chance getting a plug socket through. Or are you going to install some kind of extension where a plug from the house goes into the engine bay and the heater can be plugged in inside the cabin compartment? Like a kind of in-between bit of wiring linking the bay to the inside. A novel idea if so, haven't seen that one done before. Would be interesting to see pictures once done.
Most trailer sockets are only rated at 12/24dc and wouldn't necessarily be sufficient for what looks like a 1.5-2kw fan heater running at Mains voltage. A quick search on the net provides several examples of IP68 connectors,.and not at extortionate prices either.Another option is a waterproof trailer plug with the male fitting attached somewhere unobtrusive but reachable. Then wire the power supply to the female fitting.
This may be illegal in many places, but isn't necessarily unsafe.