Well I know this is an older thread but I come with news. I found out that you can pair a 5 button fob with the same FCC ID to the Volt and the trunk button works.
I got a fob GM part 13508769 and paired it with my car. You need both remotes that came with your car to pair more on your own. It didn’t come with a spare key but the one fob that came with my car was in rough shape so I just took the key from that and saved the fob.
Now you can double press the trunk button and it unlocks the trunk. You can then lift it up because it is unlatched.
I got a fob GM part 13508769 and paired it with my car. You need both remotes that came with your car to pair more on your own. It didn’t come with a spare key but the one fob that came with my car was in rough shape so I just took the key from that and saved the fob.
Now you can double press the trunk button and it unlocks the trunk. You can then lift it up because it is unlatched.