I know this is an old post, but I just installed my air horns. It was tough finding the space, as I think the photo of the blue ones above had a custom bracket made! (nice work BTW).
So I installed a pair of Blazer Horn dual tone air horns (BH301C). I picked them up for $30 at Autozone. They came with the horns, compressor, air tube and some brackets. They did NOT come with any wires. Luckily I had wire and spade connectors (female) at the house. It also came with a relay which I did not use.
To wire the compressor for power, I cut the connector off the small horn near the drivers headlight. I have an aftermarket alarm so this didn't matter. I ran wire to the compressor and connected it with the female spades. The compressor was mounted on the spare tire bracket, you will see in pics below.
The horns were nounted using a single bolt already on the car. I used one of the included brackets and fabricated a second out of a metal hanger I had laying around. To make the second more stable, I drilled a hole in the back wall and used a screw to hold it down:
Here is a shot with both horns installed:
Here is the final product with the compressor and horns. The compressor isn't mounted the ideal way, but it is functional... note the windshield wiper in the front of the bay. I removed it for asthetics and keep it there for emergencies!:
Here is a shot of the bracket I used... I just drilled holes at each end of it to make it work:
If anyone has suggestions for the compressor, I'd love to hear them. The horns work well and are loud as hell!!