I have one of those rubber plugs too but since it is just a pressure fit and not held in by a retaining bar I figured it would fall out or grow legs and walk away with someone! So I put in a folding step that LUND sells.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S