Did you ever resolve this issue? I want to put away the one on my workbench if you're all set.
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Art Benstein near Baltimore

Only Irish coffee provides in a single glass all four essential food groups: alcohol, caffeine, sugar, and fat. - Alex Levine

Noticed you are still fighting this issue when you posted in an unrelated thread. I'm curious about some details left out of this thread: Does the red/green wire have battery constant on it? Does the black wire have zero on it? Does the red wire then have battery when the radio is turned on? Your original post wasn't specific about exactly what the in-car conditions were on those three wires.

I still have that junkyard antenna on the workbench, and can help you by referencing its behavior if you like.
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Art Benstein near Baltimore

"I was worried that my mechanic might try to rip me off, I was relieved when he told me all I needed was turn-signal fluid."

Have you run power directly to the black box?

If applying power run directly causes the antenna to raise and lower correctly, consider adding a switch and a wire to power the antenna If you cannot find the problem.

As Dave mentioned, one of our 740s had a switch on the dash to lower the antenna when the radio was on. This was not an aftermarket switch, but a Volvo switch marked in a similar manner as the other switches.

The only reasons that I can figure its purpose is that you can lower the antenna while in the car wash and want to listen to the radio, or perhaps while listening to a tape.

Doh - sorry Dave, your answer is perfect, mine is unnecessary.

I presume you're talking about a sedan and not a wagon. I can't say as I'm overly familar with power masts in the early 940 sedans as my 940 wagons have the antenna in the rear qtr panel window and use an amplifier in a black box near the wheel well that gets it's power from the antenna mast signal, but the principle should be the same.

If the power mast now works with directly applied voltage then obviously the problem lies earlier, either in the radio itself or the wiring harness.

Rather than waiting for a reply from you, I'm going to cut to the chase and assume this is not the factory radio. If that's not the case then you're best off trying to use a meter to fault trace the wiring starting from the rear and moving back toward the radio -to aid you, there a few proper Volvo 940 wiring green manuals that can be found on-line with google.

Presuming it's a replacement radio, you need to re-examine the wiring harness adapter/splicing -something may even have come loose if it was previously working. Many radios have a separate antenna output that you would normally connect out to the car antenna mast motor (or in the case of later 940 wagons to the rear window antenna amplifier). Check to make sure that any such output is still actually connected out to power for the mast. The output signal will normally have voltage to keep the mast raised whenever the radio is powered on, whether or not the FM radio tuner is actuallly currently selected. For those radios that do not have such a signal, you need to look for another (non-speaker) output signal from the radio to tie to the antenna motor. Once such signal you can splice to is power output to a separate amp. Failing that, you would have to ignore the radio and have the antenna mast motor tied to the ignition switch (KPI), which is best accomplished by just tying into the radio on/off power supply from the ignition switch (not the constant radio +12v supply from the battery that keeps the radio memory alive). When tied directly to the ignition switch, you may prefer to run it through a dash console antenna up/down switch so you have more control of the mast. The 740s had just such a switch in the dash panel, which as I recall was next to the dash illumination slider switch. It actually was a slave to the radio mast output, but for radios without such an output it would be a next best substitute.

Hope something in the above will help point you to a solution.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now