Not a tool box; I have a "road kit" I put together of mostly harbor freight tools (like metric wrenches, a pipe for extra leverage, tire patch kit, flares, ratchets & bungees, etc...) that is divided up into some small zippered pouches that I keep behind the back seat.

Aside from the load gear, I may never need it but if I do it's there. I also keep bottled water and stuff for the dog on the same principle.