Ya make sure that box is grounded to the truck. Also do the tips of your antennas have an adjustable tip? I can't think of what that is called but there is a meter you can use to adjust the tips..shoot can't remember the term for that. Anyway if the tips are there position them 1/2way in 1/2way out. I never ran duals before, but when I had a single in my S10 I had to position it dead center to the back window to get good reception, your duels may fix that issue I had, truckers don't have issues so you should be fine.
It could be your CB too?
