On my 560 SEL with the "premium audio" (which was standard in 91) the antenna signal cable went only to the rear "brain box". From there regular wires fed the head unit.

On the antenna itself there are three wires. Black, red, white. Thanks to a member here I found that the red and black are power (as usual) and the white is the "antenna up" signal. Most modern head units of the past 15 years have a signal to raise the antenna. The harness on the car subs the white sire for a blue wire. At least in my setup.

So if you want to use an after-market head unit, one must run a new antenna cable from front to back. Not hard to do.

If you are restoring to stock, good luck! I wished to do the same but the wire harnesses were hacked and butchered beyond repair. The fader switch was crippled and most of the harness end connectors were MIA. All I had was brain box in the trunk and wires without ends everywhere.

In the end I installed my system from scratch running all new wire front to back. The only thing I reused were the speaker wires directly soldered to the speakers.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car