If you look at the schematic in the EWD for the 80 series, in the combination meter section, the schematic shows a transistor to ground with a current limiting resistor on that line. The same signal on the Engine Control page is pulling that line high with some protection circuitry added in ( a diode, resistor network, PNP transistor). The combo meter is pulling the 12V from the ECU down with a transistor that is being driven by the speedo sensor signal. So you'll need a +12V pulled up signal that pulses to ground at the rate expected by the cruise control. The signal is probably called 'approximately 12V' because it's normally high with short pulses to ground which would average to something lower than 12V.