In an IQ cart with the original Controller, the negative to the speed sensor (black/White wire) comes from the Controller Pin #13.
That is just a plain negative connection and you cut that wire near the 16-pin connector pin#13 and connect it to B-.
You can also just run a new negative wire and splice it near the speed sensor 3-oin connector.
Just to be sure, the correct voltage tests are:
Pack Negative to Speed Sensor Red wire = 15v
Pack Positive to Speed Sensor black/white wire = Pack Voltage.