Just my uneducated guess, having too many friends that drive a Merc....
Merc is VERY proprietary and their internals as well as their electronics are quite unique. Wondering if even though it is OBDII, if there is some "firewall" of sorts their ECM has in place so no tampering can be made WITHOUT an authorized Merc gauge. I know just to unlock the radio you need to have Merc access that even. I think this is the kind of reason why the Merc dealer near me is ALWAYS busy and packed with cars and full of irate customers.
Just a guess, though I can find out by giving my buddy a shout who is a tech there.
Gary Noonan
'01 S/C VX / '18 Forester XT