For anybody giving this a go...
I've been struggling with running Tailgate cables for my reverse camera and spot lighting. It's been an absolute nightmare but I've finally managed it. I did try coat hangers, cable pulling kits etc etc. But the only thing that has worked is dropping a metal nut on some string from the top grommet (as suggested earlier), then fishing the nut from the lower entry point using an Endoscope with magnetic tip.
I know this sounds like overkill but I've honestly spent hours trying to run these cables and the endoscope managed it in minutes.. well worth the £40 and will come in handy.
Link below in case anyone needs it..
Wireless Endoscope Camera,...
https://www.amazon.co.uk/dp/B08P5RZK44?ref=ppx_pop_mob_ap_share