Charles Spencer I'm not sure but I'm gonna guess it is your rpm gauge for shifting, I just bought a t680 and I have lots of questions, but of course I come up with these questions when I'm on the road,😎
I'm trying to figure out the same thing. Only mine is green. The switch is in the sleeper. I'm assuming it's for a fridge, but I don't have a fridge. The light on my dash illuminates green, not blue.....2015 KW T680