You will have probably noticed that the horn on a fortwo is rather pathetic. No one will take you seriously! Therefore, upgrading the horn to something more substantial is a worthwhile task.

If you decide to buy electronic horns (not air horns) check their rating. The current circuit supplying the horn is protected by a 30A fuse. Most after market horn kits will state what fuse they need to be protected by. We would recommend that you don't choose one with a rating more than 10A. In reality, the horn will not use this but it gives you some comfort that the smart wiring loom will cope with the increased current draw.

However, locating the existing horn is not straightforward. We found that the best access is through the undertray as it is mounted behind the radiator.

Start by jacking up the front of the car:

For safety, it is a good idea to put the car on axle stands. Try to get them as high as you can.

The undertray is secured in four places on either side. The following image shows these approximate locations:

1: Follow the undertray up behind the wheels. You will see that it is bolted high up. Use an 8mm hex socket to remove this from either side.

2: With the 8mm socket, unscrew the bolts near to where the brake lines emerge on either side of the car.

3: The final bolts are adjacent to the front jacking points. Remove these as well.

4: Finally, the undertray is clipped in to the front splitter.

Simply pull the splitter down and pull it forward. Be careful not to snag your fingers!

If you have the Brabus splitter, you will have two clips located located behind the splitter. You will need to unclip this from the front edge of the undertray.

The undertray will now swing down freely.

The horn is tucked behind where the water pipes run. The two silver pipes shown here are air conditioning pipes. Your car may or may not have them.

If you crawl under the car, you will see the back of the existing horn.

Use a 10mm hex socket to free the horn from its mounting arm. Once free, unplug the two cables and remove from the car.

Depending on the horn you have, you may want to reuse the existing mounting arm. If not, unbolt the arm using an E12 torx socket.

The arm is made out of two flimsy pieces of sheet metal.

Take your new horns and prepare some wiring to suit the kit. To reuse the existing connectors which you unplugged from the old horn, you will need two blue crimp spades (plugs).

Fitting the horns will involve an element of trial and improvement. If the kit comes with its own mounting arms for each horn, you might be able to mount each horn separately from the common E12 bolt position (removed earlier). If the arms are too short, you could try bolting the two horns together and securing these to one mounting arm. Ensure that both horns face down so that they do not fill up with any water.

Reconnect the smart wiring. If your horns are polarity sensitive, the blue wire is +12V and the brown wire is ground (or 0V).

Turn the ignition on (position 1) and test the horn. If you can, try holding the horn on for 10s. If no fuses blow and the wiring doesn't feel hot, then your installation has been successful.

Close up the undertray in reverse order to before. Start by clipping in the front splitter then refit all the bolts.