Yes, the Becker 780 can be a direct replacement for the Becker 754. Literally days ago, I was experimenting with both the Becker 754 and Becker 780 in my '79 240D. As far as sound is concerned, the two units are interchangeable. There are slight variation in the controls, but the units are pin-for-pin interchangeable. However...

If you have a power antenna, I do not believe the Becker 754 is designed to control it. If you have a manual antenna, then either will work.

As it turned out, I installed the Becker 754 in my '79 as it is the closest in vintage and I have a manual antenna.

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