Hello: I figure you probably figured this out by now, but I did just this very thing a few nights ago. I found on my becker radio, there is a plug with 4 wires, and the plug itself is numbered 1-4.

1 is BR and goes to ground
2 is sw/ge and is the acc position
3 is gr/vi and is hooked up to the park/headlight circuit for backlighting if your receiver is so equipped.
4 is rt and is 12v all the time.

I had another plug, with places for 5 terminals, but only one wire, brown went to this plug. I had no idea where this went, and my Nakamichi cd45z performs flawlessly without it. I merely taped it up. You said you had figured out how the speakers go, so I won't go into any of that. If I can halp in any way further, let me know.

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Ron Miller
1986 300E