I was going to stay out of this discussion until I had more concrete information, but have you looked into Crutchfield? I know they have several units that fit the Sprinter, but they haven't researched the Metris yet, as of tonight.
The Metris uses what MB calls an "Audio 15" in both the Metris and the Sprinter. If it is in fact the same unit in both vehicles, Crutchfield's Sprinter solutions might fit the Metris too.
I have installed two aftermarket units from Crutchfield, both Sony XAV-68BT, in two vehicles with total success. I have called their tech support once for each install, and got complete answers in all cases (quickly and in un-accented English). I would suggest calling Crutchfield. All the kits I've received from them have fit perfectly and plug in to the factory connectors. Installation instructions have always been complete. One installation I did was in a car with steering wheel buttons, which required an extra (around $40) kit to enable. Works great.
The more people who call them the sooner they will get a kit specifically designed for our vehicle of choice. I am not in any way associated with that company, which is why I do not provide web address or phone number. I don't own a Metris yet, but be assured that when I get mine, if I am not satisfied with the performance of the OEM stereo/Bluetooth, I will go to Crutchfield first.
I also concur with Greenmanedlion's advice to use an aftermarket GPS. (lifetime updates, traffic, move from one vehicle to another, program in your home), but that is an individual choice.
Some available units will work with backup cameras, which you may have to install.
PS: the XAV-68BT has been superseded by Sony XAV-W650BT.
Hope this is of some help out there.