What a great link Walt gave you to the Install Doctor website. The 740 wiring information there is for a radio with an internal amplifier, but the wire colors may not exactly agree with what you'll find.

From my wiring diagrams for late 740s, you may instead find the following for the power antenna:

For radios without a separate amplifier, the power antenna signal is a green/grey wire on pin 3 of the 9-pin cube connector. That pin number should be molded on the connector. It's the corner pin on the bumpy side below the blue wire on pin 6 (chassis ground). The Install Doctor page correctly shows this as pin C on the mating (car harness) end of the connector, but with a different wire color.

For radios with a separate amplifier, the power antenna signal is a green/red wire on pin 12 of the 12-pin double row connector. Again, that pin number should be molded on the connector. It's the corner wire above the orange wire on pin 11 (+12 volts from the ignition through fuse 14).
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now