So ive been trying to get my FT-7900R sorted in my F-150 and the coax routing is driving me nuts. I ran it under the door sill but now im getting this weird scratchy noise whenever i hit the brakes or go over a bump and i cant figure out if its the coax getting pinched somewhere or if its RF getting into the truck's electronics somehow. The radio itself works fine parked in the driveway, signals sound clean, but the moment im rolling it just gets terrible. Its on 2m mostly, sometimes 70cm.

I used some RG-58 i had lying around, maybe thats the problem? I dont know, ive done installs before but this one is just being weird. The antenna is a Tram 1185 on a mag mount on the roof. Running it off the battery direct with a 5A fuse inline, which should be fine for this radio.

Anyone dealt with something like this before, any ideas where to start poking around?

Yeah that sounds like it could be the coax getting pinched under the door sill, thats pretty common. When you close the door it puts pressure on the cable and if the shield gets compromised even a little you get all kinds of weird noise. I had basically the same problem in my Tacoma, ended up having to re-route it through the firewall instead which was more work but way cleaner.

The RG-58 isnt helping you either honestly. It's fine for short runs but for a mobile install where the cable is getting flexed and bent constantly it tends to fail faster. Most people use LMR-240 or at least some decent RG-8X for mobile work. Also worth checking that your mag mount is giving you a good ground through the roof, sometimes those little rubber feet get dirty and suddenly you lose your ground plane and everything goes to hell noise wise. Might try wiping down the contact point and see if anything changes.

check the ground on the radio itself too, not just the antenna side. ive seen installs where the chassis ground was fine at first but a loose connection would make contact intermittently while driving and it sounded exactly like what you're describing. scratchy and popping when theres vibration. just worth ruling out before you tear up all your coax routing