My antenna cap was also broken. I glued it 3 times and it would eventually break again. I saw that a new one was available for $13 so I ordered it with some other parts I needed (it just came yesterday). Search for part number 2028270031.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126