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When I remorphed my '96 truck into a pickup bed, I fitted some Bushwacker Plastic bedrail caps, black diamond plate. They looked cool for a week or so, when the right hand side cap decided to buckle out an inch in one spot. Another week passed, and the 3m tape failed in two places. One month down the road, and the caps are history, having vibrated so much as to damage the brand new paint job on my new bed. These things were professionally fitted, so they did it right but the caps still managed to damage my paint by vibrating and wearing the paint down underneath the edges. Taking them off was necessary due to the continued warping of the plastic, and the increasing damage visible as they flexed.
I strongly suggest that anyone wishing to have bedrail caps avoid the plastic ones, and either paint a bedliner over the rails, or purchase the aluminum variety. I have had aluminum caps on my other truck for 5 years, with no paint damage at all.