Easy fix. There are 3 pieces, 2 impact strip holders (plastic) and the impact strip (rubber). The holders must have their "ends" intact. The ends are sort of nubs that hold the corners of the holders into the recesssed cut outs of the bumpers. If the nubs are gone, the holders won't stay in place.

Lets assume you have good holders.

Drill a small hole into each end of a holder. The location for the hole is where the TWO holders almost meet in the center of the bumper. You will be threading some small wire through each hole, so the drill hole size is pretty small. You should use some piano wire to tie the ends together. Piano wire can be bent and has excellent tensile strength. You'll find this at a hardware store. Coat hangers, copper electrical wire et al won't work. Thin stranded picture frame wire may work. The idea is to tighten the wire together inorder to hold the ends of the holders together.

Onto the rubber impact strip. Clean the INSIDE of it very well with a water based cleaner. Car wash solution, Simple Green, 409 will all work. You'll want to scrub it well with a small brush and remove ALL the dirt. Once it is clean and dry, coat the entire inside with Silicon spray, use a generous amount.

Insert one end of the impact strip into the bumper corner cover. You'll be sliding the impact strip onto the impact strip holder while you are sliding the impact strip into the COVER. Once you have one end done work your way over to the other end. Work from one side to the other, you can't do both ends and then try the middle. You'll want to hit, hammer or kick the balance of the impact strip onto the holder. Bending the impact strip backwards a little as you feed the impact strip works well too. A plastic faced hammer works well to pound the impact strip onto the holders.

I have used a wax pencil or piece of tape to mark the center of the impact strip so its properly centered BEFORE you attach the whole thing together.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)