My problem turned out to be a bad wire connection to the aftermarket ignition switch.
I installed a new ignition switch. For the moment I've also moved the electric fans off the ignition circuit and have run them directly to the battery via a fused wire and a relay for each fan. One fan comes on using the thermostat in the radiator. The other I can control with a switch in the cockpit.
I forgot to consider the thermostat controlled fan running when the car is off while the engine is still warm, so I need to re-think this part.
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