so i've been putting off doing a proper mobile install for like two years now and finally got around to it last weekend. got the FT-857D mounted under the passenger seat with a RAM mount bracket thing and ran the power leads straight to the battery which took forever because i had to pull the door sill trim and route it through the firewall. anyway the radio itself works fine, i can hear repeaters and make contacts no problem.

the issue is the SWR on 2m is reading like 2.8 to 3.1 depending on where i park or which direction the truck is facing, but if i drive around it drops down to like 1.4 and stays there. i've got a Diamond MR77 on a mag mount on the roof. i thought maybe the mount wasnt making good contact with the roof so i cleaned it off with some alcohol but didnt really help. is this a grounding thing? the coax is just run through the door frame which i know isnt ideal. been chasing this for two weeks now and its driving me nuts

yeah that sounds like a ground issue with the mag mount almost certainly. the thing about mag mounts is they rely on the vehicle body for the ground plane and if the coax shield isnt making a clean connection to the mount base you get exactly what youre describing -- SWR that jumps around depending on where you are relative to other objects. i had the same problem with a Larsen NMO on my jeep years ago turned out the PL-259 at the radio end was barely soldered and the center pin was intermittent.

id check every connection in the coax run first. also running coax through a door frame crimps the shield over time and can cause weird SWR behavior especially if the door has been opened and closed a few hundred times since you installed it. if you can, drill a hole and use a proper feedthrough, makes a huge difference. the MR77 is a decent antenna so im betting its a coax or connector problem not the antenna itself.

I had a mag mount doing something similar on my Tacoma and it turned out there was a thin coating of something on the roof -- like a clearcoat or wax buildup -- right where the magnet was sitting. Wiped it down harder and used a bit of fine steel wool very lightly just to get some bare metal contact and the SWR dropped right down. Not saying thats definitely your issue but worth trying before you start pulling connectors apart.