Brake light switch part missing?

That is your back up light switch.

And yes you are missing the plastic post that actuates it..

Screenshot_20220131-185552.png

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)

Screenshot_20220131-190309.png
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.

Screenshot_20220131-193618.png

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