If as you say, the new Board has only be in there for 6 months, you
shouldn't have a problem returning the vehicle for replacement again.
Even if you cannot find the bill, they should have you in their records
on their computer.
If you are out of replacement warrantee, I'd call GM Customer
Service/Relations, and explain the situation to them. If the dealer
won't pick up the tab, GM CS might re-imburse you. Mark