I have been away from the C6's for a while and needed to help with a long distance engine swap. My sister in law lost a valve that took out the motor of a 2005 C6 with 45K miles. I was working on this from Florida while the car is in Minnesota. It was at 2 shops getting diagnosed and then a third shop to replace the motor that was purchased on ebay. When the car got moved to the last shop the key fobs did not work at all but a window was down and they got in with the pull handle inside. It was assumed that it was just a dead battery in the car. When the engine swap was completed the battery was fully charged but the key fobs did nothing (no doors, no click anywhere, no start). New batteries were put in both fobs, no change. All connections, grounds, and fuses were checked. Both fobs worked before the engine died. I searched the internet and forums and found no answers.

The fix:
A friend talk to a Chevy Tech that told him that you need to disconnect the battery and touch the negative and positive battery cables together for 2 minutes to drain all capacitors then let it sit for 30 minutes. After doing this the battery was reconnected and both key fobs worked and the car started up with no codes.
Maybe this is in a service manual somewhere but we spent a week looking. I hope this can help someone out when the fobs do nothing.