It should be right behind the PS rear tire kind of under the gas filler.
__________________ Don't forget to To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. first then ask questions later! The loose nut behind the wheel
The round connector is located in the wheel well as Mike said. You can buy from LR a "harness"; basically a mating connector that plugs into the connector, with individual wires coming out of it. Overpriced (until you realize it's probably hand-built with European union labor, and not some Asian high-volume mass-produced thing), but it'll save you from splicing into the brake/turn signal wires in an area that gets alot of dirt and mud.
For U.S. trailers, you'll then need to get (cheap, from just about any auto parts store), a converter that combines the separate brake/stop signals from the D-90 into a single signal for each side.