As Nick said just run the wire for the temp sender through the pvc tube at the back of the engine (used for the control cables and Yanmar wires). It will bring the wire out to the stern swim platform locker, beside the holding tank.
Remove the Yanmar cockpit control panel. You will find it sits in a molded coaming box with a hole for the wires. You should be able to easily fish the wire up through this hole from the aft swim platform locker.
If you have a problem you may have an access panel in the aft berth just below the arch attachment bolts, you can just reach this area behind the coaming box from there.
Bob