First step is to find your spare remote if you have one and try that to narrow it down. If that works, you either have a bad remote, a bad battery in the remote, or it needs to be reprogrammed. As mentioned the contacts come off on most all GM remotes we solder them back on all the time and they are usually good as new. They don't usually break loose completely until your battery dies and you go to push the new one in.. If the remote has to be reprogrammed, the procedure is usually on the back or inside of the cover. Try the spare first if neither are good and your pretty sure they have good batteries and are programmed then I would start looking towards the liftgate wires.