For the W126 body the usual location is for the speaker to be placed near the right side footwell where it can't be seen. (Although I have seen the phone speaker placed behind the center console, under the rear passenger A/C grill, but this is not a good location if you normally ride with passengers who can knock out the speaker with the feet). The microphone is usually placed by the driver's A-pillar or near the rearview mirror stalk. If you place it near the rearview mirror stalk, be careful not to jam anything between the plastic trim piece and the windshiled glass. There is a risk of piercing the rubber seal between the windshiled and the plastic and end up with water and air leaks (the whistling can be annoying, the water is worse). The electronic box for the phone is usually placed under the front right passenger floor mat. The phone cradle should be located to the right of the console, with a mounting stem long enough to allow you to reach for the phone w/o having to take your eyes off the road. Some phones have provisions for a muting function to mute the car's radio when a call comes in (a real nice feature). Finally, I would suggest that you take the phone with you when you leave the car unattneded. Too much temptation for some thief to simply break your window, rip off your cell phone and make a bunch of calls before you realize your phone is missing. Good luck.
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!