Brake light switch part missing?
That is your back up light switch.
And yes you are missing the plastic post that actuates it..
I am out right now but can get you the part number when I get back.
Google 2883211
The small post presses into the larger post to lock it in place. ( Like a wall anchor)
I dont think you will find one and they are not reproduced. And be sitting down when you see the price if you find one.
Could you remove the shift tube and have a nut or the like welded onto the tube.
If so that would be a perminant solution.
Good luck