Bad ground,high swr

I would say as long as your gounding to the bracket and a good ground on the truck it should lower your swr it did for me when I grounded my bracket to a good ground on the truck went from 2.3-1 to 1.2-1

 

Older trucks often have corrosion on all fasteners
Simply removing one bolt from the mirror where it bolts to the door or the cab,and one from a door hing where it bolts to the cab then
cleaning the bolts and all"contact" surfaces with a wire brush and sand paper.Applying a thin coat of pure silicone lube to all threads and mating surfaces will insure good contact and slow future corrosion.It also doesn't hurt to break down your mount and make sure its lubed and in good order.
Using silicone on ALL threaded connections is beneficial.Don't forget to lube the male coax prong that goes into the mount,if its not brand new then scuffing with a fine file or sandpaper will remove the discolored metal
which can then be coated with silicone.

Personally I replace my coax and mount every 2 years,I retain consistent 1.3 swr year round.Same applies to those that run a co-phase set-up(do both sides)
Using Predator brand antenna's there isn't a real benefit or
need to run co-phase for big time performance.