I got a siren! Answered
A little over a month ago, my dad got a siren on eBay. It's the Wolo Model 345, about halfway down the page. Note: if you want one, go through eBay, Wolo wants $64, but this was brand-new for $20. It's not Instructable-worthy, and I don't have enough pictures to make a Slideshow, so I'm using the Forums for their original use.
We were originally going to put this on my mom's ZAP Xebra, because it's completely silent and we wanted a way to warn walkers of her approach, but she didn't like it, so I got it to put in out Ford Courier. Since it's electric, I only go 39 in a 40 mph-zone to save range, so I'm often cut off by people going 45.
Being electric, the truck has no radiator, just a flat plate of steel. We were already going to paint it black, so we mounted the horn speaker in there.
There is an instrument bay overhead, so I stuffed the control box inside an empty slot. Eventually I'll take off the plastic case and mount it in the aluminum sheet to make it much cleaner, but this is fine for now. There were already two extra switches that did nothing, but already had an always-hot line running to them, so I wired it in to that.
It can do sirens, animal noises, or a PA. I'm leaving it set to a police-style siren, so if someone cuts me off, I can just flip the switch and give a short blast, then watch them look around for the police car. If I have more time, I can just use the PA to yell at them.
Other potential use: I'm going to make a CD of engine sounds. That way I can make my electric sound like an idling Harley at a stoplight. Or a turbine engine. Or a semi. Or a whatever.