Found this under the carpet! Looks sort of factory, there's only a resister crimped into the pin backs and a dob of hot melt glue in its wire hole.
Measured it with my mouldymeter, a Fluke copy of reasonable accuracy and got 2.3-2.4 Ohms.
Got to say, knowing this resistance accurately is super-handy, with a defeater like this I can now leave a passenger seat out with no worries about resetting the airbag unit afterwards.