The picture throws you a bit, but it is the same as on 78.
The lower cable is from gear drive on the gearbox.
Goes up into engine bay and SHOULD attach to the transducer fitted to LH side guard near booster and cruise vac unit. (As per your pic)
Of course if yours is RH Drive then the booster will be on other side (mirror)
Pic shows cable passes thru fire wall onto speedo, on my 78, it is a custom cable across fire wall to RH side, thru fire wall and onto the back of speedo.
Custom cable from a guy at St Mary's - also does clocks and instruments.
The box is the pedal box for pedals and power booster.
You need the required multi-jointed hands and arms to get in, to undo the cables, once removed, dont drive the car, I find the square drive (cable inner) is impossible to re-allign and go in. Im slowly evolving suction pads on my finger tips, to cope with such jobs.
Hopefully a pic is attached. Regards.