I have one of those key fobs to unlock the doors, trunk, etc. I replaced the battery but all that works is the horn/panic button. The power locks all work fine.
Is the fob bad? Need to be reprogrammed/replaced? What’s the easiest way to fix this? I assume going to the dealer will be costly. It’s a 2009 Pontiac so that just increases the level of difficulty. 
Check your car’s manual. You may be able to reprogram it yourself, if that’s the problem.
It’s a 2009 Pontiac _____? Depending on your exact model car, you may be able to order one from an internet store for significantly less than the dealer. You can check it at a place like:
You don’t have to buy it from them or course, but you can get the part number and try to find the best price. They’ll also be able to tell you whether you can program it yourself or not (some remotes must be programmed by the dealer, unfortunately).
The switch is probably bad.
You could try taking the fob apart and cleaning it - if it’s anelastomeric switch try cleaning the metal contacts with a rough paper towel and some alcohol, and running your thumbnail over the carbon button on the keypad membrane. If it’s a mechanical switch, try flooding it with alcohol while repeatedly pressing the switch.
