The ECM and AAM already have all of the fuel consumption information
"floating around" in their circuitry.  MBZ vehicles use multiple CAN
(controller area network) buses to pass data among the various electronic
and electrical components.  Basically, all the TC is doing is tapping into
one of the CAN's and doing some quick (i.e. $5 calculator) math with info
obtained directly from the AAM and ECM.

Of course they might have saved a few pennies in the cost of wiring by not
installing the TC harness on vehicles without the option, but the reality is
the wiring for everything in the overhead console is already going through a
CAN to the AAM (yes, even the dome lights in the ML are computer
controlled).  The very small savings in wire would be more than offset by
the cost associated with having to deal with two different harnesses during
manufacture.  In fact, almost every option in the ML is pre-wired whether
it's factory installed or not.  For example, the wiring for the heated
seats, power 3rd-row windows, NAV and cell phone is already there.

As for the installation and cutout, I'd check with an independent shop to
help there.  Either that, or a friend with some basic mechanical skills.
The cutout for the display is probably the most critical part, and it might
be possible to take the plastic fascia out and have it cut by a company that
handles plastic fabrication, such as a sign company.