Your answer may be in some of these (borrowed) pics. Look for a round roller similar to one that holds a car door open so the wind won't blow it shut.
When it gets too worn the door will not stay open. Same with shifter. It will not stay in the correct position to keep the backup light switch engaged.
If I could only find the link to these pics it would explain how to fix your problem.
Follow the bread crumbs below.
Spoon
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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut