If it's the same horn i installed (you can get them at kragen) they come with 2 plastic red or chrome trumpets and output 118db
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Here's the easy instructions for a dual-horn install..
[ standard disclaimer: i will say that i'm not a certified mechanic or electrician, and even if i were I would still say that you do this at your own risk, if in the process of installing you hurt yourself or your smart car its your own fault. if you have questions or don't feel confident that you can do this on your own, please ask a qualified person to do this for you. you have been warned! ]
step 1:
Go to the local autoparts store and get yourself a WOLO or Fiamm set of air horns. There's a two-horn package with one low and one high pitch horn, tubing, air compressor, relay, and some mounting stuff. You can get them in red or chrome plastic.
step 2: take the long length of tube and measure a 2.5" short peice, a 6" peice, and whatever is left. you now have cut the hose into 3 pieces.
step 3: open the bonnet lid by flipping the bottom grill snap things outward and lift the plastic bonnet out, place it in the holder so it stays put.

step 4: on the driver's (LHD) side below the brake fluid reservoir, locate the two wires from the stock BOSCH horn. There is one blue one and one brown. unplug these by carefully but firmly pulling the prongs off the stock horn

step 5: connect the 6" length of tube to the air compressor. If you do this first it will save you time later. Now wiggle the air compressor in past the top of the brake fluid reservoir, down and around underneath where the stock horn is located. Holding it in your hand, flip it over upside down so you can plug in the wires. Notice the polarity. Blue is positive, Brown is Negative. This is probably the hardest part of the whole install. If you look at the prongs and how they attach to the compressor it will make sense. You may need to force it in a bit if the connections are tight, but it will go in there.

step 6: Now that the compressor is connected you can connect the air horns to it. Drop the compressor down. Connect the bottom of the Y adaptor to the hose coming from the compressor. Connect the shorter 2.5" length of tube to the right Y and the longest length to the left Y.

step 7: connect the shorter horn to the short hose, and position it down so that it fits as shown in the photo. This is so you can easily close the bonnet back up when you're done, and it positions it down so that bugs and debris will be less likely to get in.
[Note: This is a good time to check the horn to make sure you wired it correctly. If you hear the motor running but no honk, the compressor is wired wrong. This doesn't hurt anything, it just means it's running backwards so wont pump out air. If the short horn honks, then you got it wired correctly and wonderful air is blowing through it.]
Some people like 1 horn, but 2 sounds more classic in my opinion, anyway.. onward..

step 8: run the longer tube under the thing, all the way through from one side to the other, as shown in the photo, then connect the larger horn to the end of it.

step 9: position the longer horn as shown so that it bends the hose into a slight U shape whilst keeping the horn in a good spot.
step 10: now you're good to go. close up and latch the bonnet, try our your new air horns!
(notice we didn't use the relay since your car has one built-in, and since this was the easy install, no mounting or screws or brackets or what not were used.)
There are larger versions of the pictures here for those that want to see them ;)
http://flickr.com/photos/miniology/s...7605889238804/