I took the seats out when I did mine (Torx 30 bolts, I think they're a 30)
Luckily mine was one of those that was already wired to the multi-pin plug.
I also run the plastic nuts up & down the thread a couple of times whilst they were out to make sure they were smooth running, plus a smear of vaseline around the gaskets to help prevent any water ingress.
Then, a new flasher unit that could handle the extra load (plus trailer indicators)
An AMR 4873 operated it all properly*, which was 3 x 21watt, plus side-repeater
Plus at times;
2 x 21watt clusters on trailer I had then (& the lighting board)
21watt & side-repeaters on the BWSOW
* On a 1998 build, with towing electrics