I found this on the net a while back. The 12 xl and 12cx have the same case.
"this is easier than it looks.
my garmin 12xl looks completely sealed but it just has a band of soft silicon sealant around that join that runs its full circumference. using a blunt screwdriver that about fills the gap ( or some other scraper with a face that is about 1mm) softly scrape out all the silicon sealant from the entier circumference.now find another object at least 20mm wide and 1mm thick .place it in the end with the AA battery battery lid and twist it carfully.keep trying at different places to crack its seal. when you do put the small screwdriver in the crack and twist again nearby with the wider object.remember to reseal with silicon and put a bit on that small rear mounting hole(on the inside) that is covered by the rummer strip with garmin written on it, because its not properly sealed by the rubber.the battery in mine is named panasonic BR 2032 ; 3volt with red and black welded wire leads on it."
Also, the internal battery can be recharged. Mine gives me the "internal battery low" message occasionally, especially when stored for a while.
I plug the unit into 12v and let it run in sim mode for a few hours to recharge the internal batt. Garmin recommends storing the unit with fresh batts to keep the internal batt from discharging.
