Just bought a 09 1/2 ton. How do you get the cap off that the trailer plug plugs into? I tried and don't want to break it.
Thanks

 

I have an 03 as you can see below, and mine has a hinged door. The hinge is at the top center of the door, and swings out and up, exposing the terminals.
Is yours the hinged type ?
If so, you could spray it with some LPS/WD40, tap it lightly, to break loose possible corrosion holding it closed. Saturating this area with lube/penetrant will not hurt anything, save it's a little messy.
I think most Chevys have this type, but, it's possible that you have an Aftermarket installation. Hopefully others here on the Forum may provide alternate advice.

Good luck.