Saturate a cotton rag with adhesive remover and clean the old adhesive off the door. Do this in a well-ventilated area, preferably outside on the driveway. Scrape the old adhesive with a plastic putty knife and continue applying remover and wiping until the old adhesive is gone. Then further clean the surface with denatured alcohol to remove residue. Don't use abrasives. What you want is a smooth, blemish-free surface for the mounting tape.
Maybe you're one of those people driving down the road with your rubber or vinyl side molding flapping in the wind. Well, for less than $20, you can fix this problem once and for all before your expensive molding strip ends up in the back of the highway cleanup crew's truck. At the auto parts store, pick up adhesive remover and molding tape. Then all you'll need is denatured alcohol, a plastic putty knife and clean rags.