Recheck your fuses under the hood. You mentioned fuse #15. On my '89 420SEL, the radio is on fuse #2. Could be they're just different configurations, but before you invest (or should I say "speculate") on a new stock Becker head unit, it makes sense to double check the fusebox.

If you're really sure you need a new head unit, and it absolutely, positively HAS to be stock, then check eBay where they go for around $50.

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Eric

1991 560SEL-Euro (214,000 miles)
1989 420SEL (Retired from daily use at 325,000 miles; Use as donor vehicle)