Problem Keeping Remotes In Sync

Both of my remotes for my 99 C5 has been giving me problems. Even after I change out the batteries and go through the FOB learning process with the
DIC they work for a few days then I'm back to unlocking the car manually. It's like both remotes are draining the batteries. With one remote (the one that's been used the most) I can't even get it to be recognized with a new battery installed. Friday night got the second remote, installed a new battery, accomplished the FOB learning process and it worked fine. The next afternoon I went out to the garage and the remote would not unlock the car. Where is the sensor at on the vehicle? Any ideas?
Jim 99 Mag Red Coupe, 6spd, Z51
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