I don't have a 240, but I used to work on them for a living. Most Nissans generally have the same type of set up electrically, which is why my answers are kind of vague and not specific.

Get a multimeter and maybe a Haynes manual (for the wiring diagrams, if they have them in there).

For power I used the main ignition signal from the heater controls also (I removed most of that stuff anyway). For ground, cleaned off a small area on this safety bar under the dash and used a self tapping screw (body ground).

Edit: I used 2 ignition signals for 3 aftermarket gauges. The cig lighter and heater control.