It's buttoned back up now but I can next time I have the cab up (passthrough is bolted back together and the grille is back on). It's easy enough to explain though...
If you look behind the grille, you'll find the lines up to the heater core in the dash. One of them already had a splice in it for what looks like a filler connection, and that's where I spliced it in (and removed that thing). It makes for long lines back to the habitat but was certainly the simplest.
There is a further thing I would do next time and will likely go back and do sometime. Splicing in serially as I did, the heat to the whole loop is dependent on having the heater core valve open. This may result in additional heat in the cab in the summer, so instead of splicing in serially, I may just T in at that junction and run the water heater in parallel from both the heater core feed and return.
If you look behind the grille, you'll find the lines up to the heater core in the dash. One of them already had a splice in it for what looks like a filler connection, and that's where I spliced it in (and removed that thing). It makes for long lines back to the habitat but was certainly the simplest.
There is a further thing I would do next time and will likely go back and do sometime. Splicing in serially as I did, the heat to the whole loop is dependent on having the heater core valve open. This may result in additional heat in the cab in the summer, so instead of splicing in serially, I may just T in at that junction and run the water heater in parallel from both the heater core feed and return.