Ooh....
Somebody must have tried to fluff that out before... (Before you had the car)
In short; it's all supposed to be going on at device 241, which is a diode-bridge in the boot fuse box, on the left.
This thing (the bridge) should look like a relay, and have five terminals.
Line RV, which is the red/green sidelights voltage, comes into device 241 as a positive feed from the column-stalk.
It should then go back out on GV yellow/green to the O/S sidelights, and GR yellow/red to the N/S sidelights.
These outgoing lines (from 241) are joined with the outputs from the parking-switch on the side of the steering column.
If that switch works, then the wiring is confirmed and you have lost the incoming RV feed to 241, from the column-stalk.