My circuit uses one relay RL1 to control the over drive, with a second relay to power the reverse lights. This second relay RL2 is a single pole changeover that removes power to the over drive when reverse is selected. Now, with no power to RL1 any damage, shorts or stuck points, the over drive can not be engaged. RL2 also powers my 25 watt reverse light enabling the gear box switch to only pass a few milliamps, so extending its life.
The over drive dash switch is a three position non latching, centre off switch with a long stalk. Moved briefly up to engage, briefly down to cancel. As you follow the circuit you will see that after initially passing full coil current to turn on RL1 it remains held on with about half the engaging current via R1 so the coil will not overheat. Switch "3-4" is the original over drive switch. The "SELECTOR SWITCH" is an extra switch, normally closed that briefly goes o/c as the gear lever is moved between third and forth or forth and third. This automatically disengages the over drive when moving from over drive top down to third when maximum acceleration is required. If not always req'd a dash mounted switch can be added to over ride its action.
In your picture of the gear box switch positions the anti rattle plunger holder can be seen mid way along the gear lever extension casting (arrowed below). The plunger, now with a ball bearing in the end & pressing on the selector shaft, trips a micro switch (mounted on the plunger holder) when passing a very small depression cut in the shaft.
For added safety all the gearbox mounted switches are wired to the negative, chassis side of the circuit, so no fire or burnt wiring!
Graham subsequently emailed:
Note that the diode needs to be connected appropriately depending on whether your car is positive earth or negative earth. However in my tests with the LH overdrive the solenoid would not reliably operate with 25 ohms in series, and would not operate at all with 35 ohms in series. The earlier D-type overdrive requires significantly more current to operate so is even less likely to operate without a diode. So being powered with Graham's 75 ohm resistor is series should not cause any problems, but then neither will putting in a diode to be sure.
I also strongly recommend fusing the ignition supply to the circuit, as I do to all overdrives (and fuel pumps). Graham tells me uses fused relays, but as far as I can see these are only available in the basic 'on-off' flavour i.e. RL1 and not with the 'changeover' type RL2, which does leave some components and wiring unfused.