If you are referring to the ceramic fuses, you can get these at Canadian Tire. To check your power, and mind you I have a W126 (85 380SE), not 124, so these might differ, you might consider tapping into the cigar lighter wire to bypass all the old Becker wiring which sounds like it is suspect anyway. Use your tester and check that you get 12V with key to IGN, none when key is off. I think that is the case with the lighter wire. If this works, it will get your Sony powered up for testing.
Also, recheck the black/yellow, should be on/off with IGN, touch multitester - to brown, and also to a clean metal body point, like where all the brown ground wires are ganged together. The + lead of course is touched simultaneously to the black/yellow.

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1986 560SL
2002 Toyota Camry
1993 Lexus