Hopefully someone can get you what you need, but there’s virtually no wiring “under the seat” on my own T3 Cali - it’s mostly behind the RH side panel, in my case. Having said that, the chances of a T3 having survived without the owner(s) having relocated a component or two, or replaced/rerouted some wiring are slim indeed.
Greg Bender’s looms are renowned for being “right”, so you might be better off fitting the loom, then seeing where components such as horns can be fitted and connected.
If you aren’t already aware, there are some first-class wiring diagrams available to members of MGCGB, and which you’re going to be grateful that you acquired!
If there are specific components that you would like to see “in situ”, just shout: there’s plenty of owners can show you their variations on a theme, and from which you can choose. Hope this helps.