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This connection (on the speedo) actually feeds both cc and engine ecu with speed signals. It's a digital, square wave (or pulsed like you describe it) signal which will probably work fine in your application as long as the nav speed input is a high impedance, passive type. The connection must not be grounded during any circumstances.
Frequency is wheel sensor speed divided by four. Should be close to 10000 pulses/mile (6150/kilometer). Unfortunately, the exact figure isn't stamped on the speedo face like the 240's but if your into math, it can be calculated on wheel circumference basis. Wheel speed sensor produces 48 pulses/wheel turn and cc/ecu output consequently 12 pulses/turn.
If your not sure about nav input characteristics, tapping directly into the speed sensor wire is fool safe but this on the other hand is a rather aggressive analogue signal. Frequency as described would be four times higher.
Someone mentioned this subject regarding a later (1997?) 960 one or two years ago. Apparently that car had a dedicated speed signal tap in or close to the original radio wiring, probably because RTI was introduced as option that year or maybe because some radio units offered automatic volume adjustment proportional to vehicle speed. Anyway, checking the possibility before you get behind the instrument cluster couldn't hurt.
Martin
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'65 121, '73 165
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