Nice sprayer set-up! Mine isn't near as nice, it's just a sprayer tank made for a 4 wheeler rack. But it sprays what I need.
I started with a direct, wired to the battery harness. You've seen these same ones at Walmart, in the battery section. They have 2 ring terminals, and a 2 wire flat connector. And it's easily made from scratch. Mine came with the charger, so I just used that.
I took that simple harness, and added a fuse, and a switch. Mounted the switch behind the seat. Routed the wire so the flat plug comes out behind the bottom of the back window. Just above the rim of the bed. When the sprayer or whatever, is used, it has the same plug.
Mine is first, and foremost, to keep the battery maintainer connected anytime it's parked in the barn. But it's also a direct, fused, switched circuit, for direct battery access. Suitable for any 12v needs and accessible from the bed.
Extra 2 wire flat plugs are easily found in any auto parts electrical section. So you can literally hook up anything 12v. Plus you should always have a battery maintainer anyway. Works very well for anything 12v. you want to plug in.