JimG,
In looking at the schematic, one can just replace the bulbs vs. having to replace the entire thing. This is consistent with my experience with MB light fixtures as I recently had my mechanic replace the main bulb on the 2006 MB wagon and I had a similar concern. Just a bulb replacement that cost me $40, mostly labor as it was bloody difficult to get to it in the engine compartment.
The rear hatch on our B250s is easier to get to. Figuring out how to remove the back of the entire thing is the tricky part. If you have a problem doing so, let me know and I will ask my mechanic. I need to drop my B250e off this winter so he can replace the front end damaged when someone backed into it when it was parked on the street. BTW, the insurance check was for over $5K to cover a new front bumper, left quarter panel, new left front bulb and of course labor. I can ask him to show me then.
Good luck.