Hi Len,
Will you be at Pagodafest Chicago?
If so, I can show you then, because I'm not sure what issue you are having.
Mine slide on and are secure.
This might be the key. When I first installed them, the hooking "loop" was pointed down, and everytime my seatbelt got pulled tight, the plastic part would pop off.
Then I mounted it with the hooking loop pointed up, and now it works just fine and stays in place.
Good luck !
Mark