1995 E320 Magical Radio Antenna

The radio antenna has a mind of its own. Sometimes it will go up when the radio is turned on, sometimes it won't. If it goes up, it usually
will go down for no reason. I have verified that mechanically the antenna mechanism works fine, and am suspecting the signals that the antenna receives to tell it to go up and stay up. As far as I can tell, when there is voltage on one of the connectors at the antenna, it will go up and stay up until the voltage drops.
Since this car came with a phone, I presume that the "antenna go up" wire from the radio is also connected to the "phone off hook" wire. If there is an intermittent connection somewhere in that daisy chain the "up" voltage of course drops to nothing and the antenna goes down.
Can anyone suggest trouble shooting this? Where does the phone's "up" connect with the radio's up? I have removed the phone, so bypassing the phone's "up" can be done.
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