The panel under the driver's side column does come off with relative ease. There are a number of screws to undo. Also, there is a screw under the parking brake release (have to pull release to reveal) too. Getting the panel out of the hood latch release is the only pain in the @ss. You might not need to though because you can still access all of the wires and such from there. I am sure there must be a positive constant power under there, but not sure which one or where it would be.
As for running the wire... I don't know exactly how this would be done. I do have the mic to a speakerphone mounted up there, and it looks like it runs under the ceiling panel to the A-pillar, but don't know how that would work.
Good luck and I'm sure others will be along and provide their expertise.

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)