Hook, you'll see from the first photo that the plug is 'over-engineered' for the task, since it's over-large and has a safety clip to stop it coming undone. I haven't seen these for sale and although it might be possible to find one on the internet, it would take time. I suspect that the plug won't be available as a seperate part from dealers, but will come with the wire attached and probably the lamp reflector. You could however try Rutter's - they are good at sourcing obscure bits.
If you chose to plug/unplug your extra lamp the way you suggest, you'll have to take the spare wheel off to get at it. Have you considered that?
This is why I went for the spliced in 'umbilical cord' which is reasonably easy to fish out when required. It's not difficult splicing the loom, but it does need soldering for a lasting job.
