I did a Google search and found a Mercedes/BMW recycling website,
myrecycledparts.com, that had the part, #1408200191. It was being sold through
eBid Online Auctions - Buy & Sell in our Fee Free Internet Auctions for United States, which I never heard of before, but the sale went through fine and I have the part in hand.
I haven't installed it yet as it looks like I need to remove the antenna and lower seal so the upper can thread onto the lower. However, the part is correct and I can see that it will fit once I figure out how to approach it.