The relay mounts to the rear surface of the battery box, under the dash. Here's a (poor) scan from the workshop manual/Bentley.
The OD wiring did not originally run to the fuse panel & regulator. Instead, the power for the solenoid comes from the back of the ammeter (battery side, oddly enough), while the power for the relay coil comes from the white wire terminal of the starter pushbutton (through the operating switch on the dash). Then two wires go from the relay to the gearbox, I believe under the carpet and through a grommet on the LH side of the tunnel. One from the coil to the lockout switches on top of the gearbox, and the other from a contact to the solenoid.
I know, several sources show the operating switch on the ground side of the relay. But what I've described above matches what I found on TS39781LO (with original-looking wiring in sub-harnesses), and makes more sense than the scheme given in the book (which requires an extra run of wire from the relay to the dash).
PS, it also matches what you can see in the photo