I noticed that the B-Class Electric Drive from 2014 I bought doesn't have the Range Plus button on the Multi Switch Panel. I found out this was the only year it was in an optional feature.
I began to wonder what exactly they would do at the factory to enable this option. As a thought experiment, it seems likely that the option comes from the button being present and connected to the wiring harness. It would be cost prohibitive to create versions of the wiring harnesses for cars that have or don't have the option. So, my guess is that the wires to connect the button to are there hanging out behind my panel.
The other indication that the button must be the key is that when looking on the MB parts site, no Multi Switch Panel part is available for 2014 (almost nothing is) but the 2016 parts (yes each combination of features like heated seats and parking assist, or lack thereof have their own unique part number), and the 2016 parts say they fit the 2015, even though 2016 was the refresh year, and the 2015 and 2014 appear to be identical except for Range Plus becoming standard in 2015.
So, as far as I can tell, the button is the key.
One caveat.... I looked through virtually every part in the MB parts site, and the instrument panel has a curious note about "With Range Plus pkg " and "Without Range Plus pkg." (for a single part number). It seems to be saying that the part will work both for models equipped with Range Plus option and models without it. I thought this was a curious distinction to make at all, and think that this might also be key in enabling the button once connected. Maybe the 2014 models didn't get the same Instrument Cluster originally if the option wasn't purchased, and because they made it standard, the part number for the Instrument Cluster without Range Plus was discontinued and replaced by the "enabled" IC that can function either with or without the RP button???
So... I bought an Instrument Cluster and Multi Switch Panel from a 2014 unit with the Range Plus package being parted out on eBay. The parts are on the way Monday, so I'll update this after I get a chance to do some surgery.
This also makes me wonder if getting backup camera functionality might be as simple as plugging in an OEM backup camera to the wiring harness? Who knows?