Looking for the car speed signal for the radio

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I'm installing a new GPS/audio head unit that has a connection for a "speed pulse" to aid in GPS accuracy. The factory radio has a speed sensitive volume control, and I believe the signal for the car speed to the factory radio is a plug with 2 wires that the wiring diagram says is "data" with no further description. All the other connections to the radio are clearly described and relate to audio or power or the steering wheel controls. The wiring diagram shows plugs C240a, C240b, and C240c for the radio. C240c has two wires labeled "Standard Corporate Protocol Data + and -" I seek confirmation that these are the speed signal, and that it would be safe to connect the Spped Pulse Input connection of the head unit (Pioneer AVIC-6400NEX) to the data + pin of that plug.