There's a cable from the shifter to the back side of the column with locking mechanism as part of the anti-theft. If you can't pull the shifter out of park, you can't unlock the column. The button and brake are both required to release a solenoid that holds it in park so you can't start it in gear (safety requirement). The button is a mechanical linkeage to farther down electrical switch. I'm not sure if the brake light swithch also powers the shifter logic or if it is separate (without going to the shop manual). Makes sense that it would but GM isn't always so inclined. Getting the console apart to get at the components is something of a chore. You'll need some good snap ring pliers to remove the snap ring below the button ( which just pops up with a some careful prying). Once you get the knob off, the console glove box top, screws (metal and plastic types including some under the coffee holder), hatch button, the top of the console comes apart from the back in three pieces. You'll then be able to expose the shifter and cables and soleniod. You'll need to jumper to the solenoid power lead because it won't move manually, it's pretty strong. That will release the shifter (if it energizes). If not, changing it is something of a chore but possible. Finding the short you'll be on your own unless it's just the fuse!