Accessory circuit: you'll have to chase the circuit with a test light or a multimeter. It may help to remove the toolbox sheet metal and the floor pan (under the operator's feet); this will give you access to most of the wiring from the dash back. There's a big connector for the rear harness under the operator's right foot.
No lights in the gauges: There is one bulb which provides light for all the gauges through fiber optic cables. The housing for the bulb almost resembles an octopus under the dash.
No power at the lighter: could be a bad wire to the lighter, or the wiring harness "modified" and the lighter bypassed. Just have to look at it.
Fuel gauge doesn't work: on the Modern View, you can search this forum and find lots of helpful advice addressing non-functioning fuel gauges, including how to ground the sender. You'll also find information covering matching the resistance of the sender to the resistance of the gauge.
Half the headlights light: I would suspect the non-functioning lights have lost the ground path, as they ground through the fender, through the mount, and then into the axle housing. First, I would check to see if the lights are getting voltage to the terminal leading into the light housing (it's fast and easy and tells you quickly whether the issue is a voltage issue vs bad bulb/poor ground.) It could also be a bad light switch, because the two outside lights connect to a different pole on the light switch. Is it both lights on one side, the two inner or outer lights, or none of the above?
Let us know what you find.