SL/R129: 12v. power point
You need to go to the dealer and get the "keys" to take out the radio and heater unit (under $10), then it is an easy matter to remove the ash tray, remove the phone box (screws in back), drill a hole in the back, route the wire behind the dash, and tap into the 12 volt power at the cig. lighter.
I installed a Sirius radio this way, and put a 12 v. under the passenger side of the dash board. See this thread:
https://mbworld.org/forums/sl-class-r129/143178-here-s-what-i-did-box-dash.html
I also have a radar detector, but I just use it on long trips, so I keep it in the center console and only put it out when I need it.



