Hello, Welcome to Just Answers! I will assist you with your question.
I can give you a reset procedure, but first I will need to know where is your present location (state) for the proper location code.
I was able to upload the illustration:
its in co. and set in zone 5 we have reset it twice and same problem
the dealer cant fix it
there is nothing added to the truck all stock and this is the second one 2011 same problem i think that it is in a maged pull from some thing
there is no comp in the mirror its on the steering wheel.
i mean comp button
This is the only reset procedure I am aware of. It sounds to me that the compass its self needs replaced. I do not see any reason the dealer would not be able to fix this problem, if they refuse you may need to find another dealer or contact GM directly to make them move on the issue.