JLR, my boys in my installation department utilize the aftermarket handfree kits with MB systems all the time.
They can wire them to the preexisting antenna, radio muting, and honk the horn when you are not in the car if you like! It can be done. I know my guys charge about $165 to just install it. It is $30 extra to add radio muting, antenna, etc.
MB's are so well put together they are harder to take apart! That is why it is expensive. Check with a reputable wireless provider in your area. Some do installations and some do not. Check around and beware of Circuit City they are cheaper but they usually forget some things like hook up the antenna or drill a hole in you roof.
My boys just installed a nokia handsfree in my parents new SC430 convertible and they used an antenna that you cannot see! They run it up the inside of the A-pillar and it works superb! Who wants a ugly antenna on a 02' Lexus or a 00' CL for that matter!
If you want to come here and do it cellphones is my game and Sprint PCS is my name.
13' T & C minivan 90k
16' golf sportwagen 900 miles
previous MB units..
01' e320 171k (regret sale)
91' 300d 2.5t 300k (sold to ecoofidaho)
79' 240d 177k (old yellow my avatar)
87' 300d 169k (junk but fast)
85' 300tdt 261k (RBM parts delivery)
85' 300dt 68k (one owner cream puff)
82' 240d 250k
84' 300dt 198k (sold to diesel don)
91' 190e 2.3 61k
88' 260e 140k