I also wanted to add a momentary contact type switch for my Valentine-1. I specifically wanted it set up so that when the key is turned off, and then the car is re-started, the V-1 does NOT turn back on...In case you're wondering why, it's because of the irritating 'startup dance' that the V-1 goes through at each power-up. I don't use it all the time, and hate listenting to that screeching every time...
How long is the irritating screech? If your car still has a 'fasten belts' timer, you could plumb it into the V1 such that it mutes it until later. Or, just put in some kind of muting timer that's unconnected to the car. When I used to drive with my 'equalizer', I got a ticket or two when I might not have, had I only had the thing turned on! After that I hard-wired it into the car so there was no manual step required. -- Jim