I have a 2004 f350 diesel and my tail lights
go dim and when I turn my signal on both
tail lights and plate lights flash checked
flasher and grounds and replaced mfs
already and it gets worse when I hook my
trailer up help
Try cleaning the light bulb sockets and contacts. Are you saying this is only a problem when the trailer is hooked up or all the time? You could check the turn signal sending unit. If this is just when the trailer is hooked up then the trailer wiring is messed up.
Search Ford F-350 Super Duty Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford F-350 Super Duty Experts