Here's the story on the placement of the o/d electrics.
The O/D relay goes on the firewall on the drivers (left) side of the car
to the right of the heater if your looking towards the rear. The Vacumn
Switch goes below that. The hose from the Vac switch goes to a little
nipple on the intake manifold. The wires from both snake along forward
of the heater and then down to all the connections leading either back
aft or to the dashboard part of the harness.
1958 MGA (RHD)(mine)
1964 MGB (RHD)(mine)
1979 MG Midget (hers)
1980 MGB (ours)