I guess I've been feeling motivated to do things lately.
The intake heater issue was weighing on me, and since winters coming, I finally decided to just order a new one. In an act of pure retail therapy, I used Partsouq, paid too much, and it came across the entire planet in just 3 days.
Initial tests showed resistance to look identical, showing as basically a dead short, but the old one had some telltale melty bits right at the terminal which didn't bode well.
I don't know the status of the heater relay, since it appears to be located under the primary battery, so when considering pulling that all out, and realizing I would need to reset my clock I said **** that and stopped investigating.
Installed the new one and tried it out. It still draws like 150A+ initially, but it dropped in seconds, much faster than before. After about 5 seconds it was at around 50A, continued dropping steadily but slowed considerably when it got into the 15-20A range. When truck starts, it then climbs, but then remained steady in the 60-65A range.
So, as
@DancesWithMutts had mentioned earlier, resistance is changing as the element heats up. When the vehicle is not running and has no air flow, element gets hotter, the current keeps dropping as resistance increases, allowing the heater to self regulate. When the truck is started, the incoming air is cooling the heater, increasing current, and it seems to hang around pulling 60A in 20c ambient.
The shorted terminal on the old unit meant I was just dumping electricity into the frame, so it's good I've had it disconnected for so long.
I'm now looking forward to some less smoky starts.