I finally dug into mine, 2022 579. The factory antenna system is garbage. The factory system uses cophased system. There does not seem to be a duplexer in the system like cascadias use. The factory mounts you can remove from the outside. 10mm socket for the nuts. Thake care not to let backing plate dislodge from the holes, can be a bear to fish out.
The coax is attached to the mount with mini PL-259 to Mini SO-239 on back side of mount. There is a grounding braid from the mounting plate to upper door frame, they do not remove paint. you may want to take it loose and sand to bare metal to be sure it is grounded. I found it easier to use fish tape from the out side to pull new coax thru. I decided to single out the antenna, Less headaches. If you use premade coax with Pl-259"s on both ends you will need a SO-239 to mini PL-259 adaptor or order a coax with PL-259 on one end and a mini PL-259 on the other..
After installation I put the factory antenna back on, lowest SWR from 11 to 28 mhz was 3.4 to 1. Put a K40 tunable tip 4ft on was slightly too long. Trimmed 1/2" SWR came down to 1.2 to 1. I disconnected Passenger side to try a VHF antenna so far haven't had luck with that. Disconnecvting the coax from both antennas seemed to have no effect on am/fm or xm radio.