Below is a photo showing the pinouts on the cable for your SP 26xx GPSR. Pins 3, 4, 5, and 6 are only functional on the units that support dead reckoning (the 2650 and 2660).
It is important that you fuse the positive DC power being supplied to pin 1. The cable has a 'male' end on it, so to speak, and it is possible for the pins to short out to ground if the end of the cable brushes up against the handlebars, brake lever, steering head, etc. when it is not connected to the GPSR.
PanEuropean