Internal to the Jaguar, the white wire (which contains the PWM signal) is put through the diode of an opto isolator and the current returns via the black wire. As pointed out, the red wire isn't used. The other side of the opto isolator goes to a GPIO pin of the microcontroller in the Jaguar which measures the pulse width.
You can get the Jaguar's schematics which might provide you more insight and help with your interfacing exercise.. The CD accompanying the developer's version of Jaguar are available for free download on the TI web site. See
http://focus.ti.com/docs/toolsw/fold...-bdc24-cd.html
Also be aware of the pulse width and frequency requirements. By default, the Jaguar uses a wider pulse width range than standard hobby servos or the Victor. NI and TI (then Luminary Micro) worked together to provide more granularity to the output and therefore the PWM width is wider. It can also be operated with a period in the range of 5-30 ms (5 ms for a 200 Hz update rate). If you need it to work in the same range as a standard hobby servo, you'll need to calibrate the Jaguar, and remember to recalibrate it if you put it back on an FRC robot later.
-Scott