I'll offer insights based on working with amateur radio in mobile installations.
In the case of a 12-vdc power outlet I wouldn't use it for anything other than perhaps charging a cell phone---and at that be concerned about power fluctuations and possible damage to the electronics (device, not tractor).
12-vdc power outlets on vehicles are not known for electrical circuit (contact) stability. The male plugs are not reliable (in the sense of maintaining constant electrical contact (vibration)), the sockets get dirty resulting in high resistance and for that reason increased heat and blown fuses or melted wires.
There is a whole host of known problems. Although not what's under discussion here, it is pertinent that in the amateur radio world the concensus is that the 12-vdc power port or 'cigarette' lighter should NEVER be used; it's mostly decoration.
I sure wouldn't chance an expensive 12-vdc sprayer motor or high-output lamp.
1. On a tractor (on later model 'plastic body' and hybrid vehicles a few differences) run a red (hot) and black (ground) wire directly to the battery terminals.
2. Fuse both wires within inches of the battery.
3. Use 12- or 10-gauge SAE-rated automotive stranded wire.
4. Seal all crimp connections (RTV or fingernail polish, hot glue if not too hot an area).
5. To traverse the engine compartment (hot engine components) run the wire in corregated split loom.
6. Use Anderson Power Pole connectors on the loose end for power access (with these connectors properly installed on the power supply wire and powered device(s) anything can be powered with no fear of reversing connections (they're keyed)--and connections can be 'stacked' for multiple access points). APPs have become common in mobile power applications (think oversize stereos in cars with dark windows driven by deaf kids wearing sun glasses at night).
If done right the wiring install will only have to be done once and the installation as described will last the life of the tractor or your interest.
It's odd this question arises in this Forum at this time; the exact same question came up last night in a class for amateur radio licensing spouse and I are assisting teaching.
Please post back your continuing experiences so we may all learn.