Next, using an automotive cable splicer (available from Supercheap, Repco, Bunnings) splice in a short wire and solder in a 470 ohm resistor. Then slide heatshrink tube onto the wire from the diode and solder that wire to the other end of the resistor. I forgot to take a picture of the heat shrink:
Clean up your mess with zip ties and split conduit:
At the solenoid plug, fold the diode alongside the plug. Insert the plug, resistor, diode into a snug piece of heatshrink and shrink it into place. This will stop the resistor from simply falling out. (No picture - it was too tight for space.)
Finally, think more about the wire route than I did! See below. I should have gone under that one, but it was too late when I realised.