Right behind and over the brake master cylinder area, in toward the center of the car, you will see a large rubber plug with a number of rubber nipples of various diameters sticking out from the plug, and some wires and tubes porting through several of the nipples. There are a number of unused 'ports' - piercing a hole in the appropriately sized nipple, slightly smaller than the diameter of the wire (or vacuum line) that you want to route, will get you through to behind the dash.
Under the carpet and removable pad there are wiring channels leading to the rear of the car. I remember an older article talking about removing the rear seat back and bottom cusions and looking for pass throughs in the rear wall, but i don't remember the details. It is probably in the archives if you search under rear seat removal.
(Try
this link)
Hope this helps.
__________________
John
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E