1997 gmc sierra battery wont stay charged. Alternator and battery have been tested. What else could it be...fuseible link. If so where do I find it?

I suggest you hook up 12 V digital multitester to the battery running & get your voltage. Next stop the truck & get the voltage leaving it connected & you should see the voltage steadily dropping. Then, take off the Negative cable & hook up the tester again to the 2 post of the battery & see if the voltage drops as before, or has stabilized. That will tell you if the truck is draining the battery for sure of if you have a bad cell. If it is the truck, you will need to go through troubleshooting the entire vehicle by eliiminating all use of current and gradually hooking things back up. IE: disconnect main fuses/breakers & see what happens on the tester. Then start connecting 1 item at a time & see what is requiring power or if you have a short. You should eventually find the culprit even if you have to remove fuses and test each time. An additional tester that test amperes without penetrating the wiring is helpful in finding the drain. Normally in the auto-parts stores. I just lay mine over the wiring & follow the voltage until I find the wiring leading to the one using the power or that is shorted. Good Luck.