Thanks @California Rob @dspuk @This Eva and everyone else who posted, managed it in the end…
I'd almost lost hope to be honest, no matter which way I tried to thread past the air vents from the front, my cable feed rod would always block.
I ended up using the dash cam itself (Garmin Mini 2) as a makeshift camera probe…realised in the end, that there isn't that much on the other side of the air vent, when looking from the rear.
In an inspired move,
I fed the cable rod from the rear, making sure the lead flexi rod was fed into the correct channel. It slid down and popped over the air vent with barely a push.
I'm over-the-moon as I've been trying unsuccessfully on-and-off for about two weeks. Even started to consider wiring a new 12v socket into the leisure battery as I couldn't manage the cable feed.
Getting the cable from front to back now meant I could pop off the grab handle, and the flappy-downy handle around the door and feed it through the headliner, down the A-pillar and into the trim where my ODB power adapter is (LHD).
Two sad parts:
1.
The cams don't run from the Leisure battery, but the engine. Why have two AGM leisure batteries in there if I have to connect to the engine battery and hope it doesn't run flat. To be fair, I don't reckon those two mini cameras use so much power while they're in parking guard. Plus, the ODB adaptor cuts off if the voltage goes too low. Still, running from the leisure might be worth investigating in future.
2.
I really didn't fancy attempting to disassemble the rear tailgate trim to run the cable nicely. It would have been my ideal, but the process of taking it all apart seems so difficult (too many breakable plastic clips) I didn't feel it worth while.
Now have two Garmin Mini 2 cameras installed (mostly) properly!