TheDon is right - it's the plastic that 'ages'. While I've not heard of using aluminum polish, it would probably work real good. There are also a number of plastic polishes available at your local auto parts outlet that can also be used. Cloudy headlights are common on ML's. Polish the exterior of the lens and you'll be amazed at the transformation.

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MBCA Member #B012089 (Lone Star Section)
OBK Member #47 (W123 Division)
'96 SL600 (105K) Triple Black - Mein über-Fräulein
'79 240D (292K) Yellow/Saddle - Mein Spielzeug
'01 ML430 (123K) Black/Saddle - Wife's Ride
'94 SL500 (164K) Green/Champagne - Daughter's Dream
'73 450SL - RIP
'86 300E - RIP
'88 420SEL - SOLD
'94 S320 - SOLD