You mean they are getting cloudy on outside? If so why don't you just clean them up, thats what I did, wet sand with 1000,1500,2000 grit and apply plastx using corded drill to buff and wow they look like new!

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2003 BMW E53 X5 4.4i - M62 engine (Non Sport with Premium and Cold Weather Package)
Color: Steel Gray Metallic
Mods:4.6is Exhaust (Pipes, Mufflers and chrome oval tips) BMW Sport package Taillamps, BMW Tire Valve Stem Caps
Medium tinted front windows
JVC HD Radio
Alpine 4x100 channel Amp
Orion 600W Mono HC Amp
Three 12" Sony Xplode in subwoofer enclosure