While it seems totally off the wall, our old 2006 SRT8 Charger had the same smell, but only when heat was on and only for a minute or so, like you.
And it may have been sporadic, like not all the time, and the last year or 2 never noticed it.
I swore there were a couple crayons that got shoved down the defroster vents or something, but never found out what it was and never had a problem.
FWIW, I just googled it and Mercedes of the same era as our old Charger (which was a Diamler/Mercedes chassis) had a crayon smell and it was some wax in the insulation somewhere.
2016 Ram 2500, MotorOps.ca EFIlive tuned, 5โ turbo back, 6" lift on 37s
2017 Heartland Torque T29 - Sold.
Couple of Arctic Fox TCs - Sold