making a negative pulse

How can i switch a constant negative output to a pulsed negative output?

Use this link. In your case though, 87 would go to ground, 85 would go directly to ground, and 86 would go to constant fused 12v+ with the capacitor and resistor wired in parallel on it. 30 would give you a momentary (-) output.

Also, if you wanted it to pulse every second or so rather than stop after the first time, you could use a flasher instead.

Chris, would the relay setup you described above work for what I want to do in the post below that I made in the Car Security Forum?
Forum question link from Car Security

Also, if I want to make it pulse for shorter than .5 seconds, do I lower or raise the capacitor value? What would the value be if I wanted a .25 second pulse? Lastly, I have some spare parts in my electronics bin but I don't have a capacitance meter. Do you know what the markings would be on a commonly found brown ceramic capacitor? Thanks, for any help you can offer.

-Steve