The arduino may seem like over-kill but the system also needs logic like:
-turn off after 30 seconds so as not to drain the battery or annoy the neighborhood after the spotties are long gone.
-keep alarm going for 30 seconds if spotttie is immediately plugged in.
And besides, once I wire in an arduino, I have a platform ready for other car related projects.
I appreciate the diagram heaps boffin1; I am starting to get an idea of how to approach the problem now. For some reason I hit a massive mental blank approaching this problem and this stream of ideas has cleared that, so again thanks