Although my previous solution for testing LEDs basically worked, I wouldn’t go any further than that.
Because this will need testing more as I work on fitting the lights I came up with a better solution. This uses a USB plug and some properly attached female spade connectors.
This means I can now use a USB power pack as a source of juice and it has an interface that mirrors the wiring loom in the car. Admittedly the car runs on 12v rather than the 5v of USB but, whilst it won’t power the old incandescent bulbs, it is sufficient to light an LED.