we did it without a rivet gun and it was very difficult. I say "we" because you really need 2 sets of hands... the rivet gun is really worth saving the frustration.

if you must, heat up the plastic rivet a bit with a hair dryer so that it can bend a bit easier. Use pliers to pull the stem outward (it will take 1 person to do just this) while the other person uses any other tool to hold the small base of the rivet in place against the flare. Like I said, this was the only way we could figure out how to do it without the gun... very not recommended. We gave up after putting in only 3 rivets under each wheel. Body shop eventually put the rest in without being asked when they saw the extras in my trunk.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr