Found it! The truck has been sitting in the rain without being driven for the past 48 hours or so. Water is entering from the rubber grommet in the firewall where my SwitchPros cable was passed through into the cabin. I felt all the carpet/jute padding above the grommet and it's all dry so I am 99% confident that grommet is the source of the leak. Should I just attempt to seal it up with silicon RTV?
Found it! The truck has been sitting in the rain without being driven for the past 48 hours or so. Water is entering from the rubber grommet in the firewall where my SwitchPros cable was passed through into the cabin. I felt all the carpet/jute padding above the grommet and it's all dry so I am 99% confident that grommet is the source of the leak. Should I just attempt to seal it up with silicon RTV?
Yes. Just goop some RTV all around it (while wearing nitrile gloves).
I'd suggest not getting the adhesive type, but the sealing RTV, in case you need change anything through that port.
On the GX470, both main harness bulkhead grommets though the firewall have a little "nub" on the top of it that one can snip off and fish wires though. I have two 10-ga wires and a half-dozen or so 22-ga relay trigger wires though mine on the DS, then a GRMS antenna and locker wiring on the PS grommmet. It's nice as it fits tightly around the wires - so there are no leaks. See picture below (4th gen 4Runner from the internet, but my 470 is the same).
It looks like here that the middle of the grommet was sliced and the wires were fed though, which definitely could result in a leak. Or, perhaps a different grommet was used for the fishing. If the GX460 also has the nub on the main bulkhead grommet, I'd suggest slicing it and fishing the wires though it instead, and then repairing the cuts in the grommet on the firewall side. If it does not have the nub, I'd use a bunch of RTV around the wires and grommet (on the firewall side) as @r2m has suggested.
RTV would work but I like Rednexus idea. I've used a lot of RTV in my day all types, oil resistant, high temp, non hardening, etc. mainly on flanges of pumps and turbines. Technically coming through the firewall you would want fire resistant\proof type but I wouldn't sweat it personally.
For cables and raceways on ships we always used duct seal for bulkhead penetration, and I would in this case as well. On both sides of the bulkhead, it is non hardening so removable down the road and as long as the water is not pressurized will keep out water and smoke. Not a fan of the sliced grommet in the video and if you keep it in the same location would pack it with duct seal from both sides well.
Sea (shore) story time. We always called duct seal monkey chit. Lot of things had the name monkey in it in the Navy, penetrating oil came in a green can made by the cheapest bidder we called it monkey piss. When I first retired from the Navy went to the Ace Hardware store in town and wasn't sure what to call monkey chit so asked the guy for some monkey chit. The young guy gave me a blank stare but an older guy working there heard me and said oh you mean duct seal and walked me over to it. Apparently HVAC people also call it monkey chit as well.
With both sides you might trap water in between the two, where it could degrade the harness. I'd suggest one side only, on the leak side. Then you can also see if it is still leaking.
