OK, seemples.
First of all a linky to the w202 diagram I have. The 208 hopefully shouldn't be too different.
Linky
Check at the horn relay to see that you have...
Ignition live (shows this as pin 3 on the w202)
You can also check that the relay is working by listening to see if it clicks and putting the testlight on what is shown as pin 1 when you press the horn.
If there is no supply, I would suspect the fuse. If there is a supply and the relay clicks bit no output suspect the relay.
Now, assuming that you have a feed, you may be able to activate the relay with the testlight. push the probe into pin 4 and the light should come on dimly. It MAY activate the relay and cause the horn to blow. Careful - you WILL jump and hit your head on the bonnet.
Let me know how it goes to there and I can take you to the next stage.
Apologies, this has been written in a hurry, and I will be unavailable for the next couple of hours.
Remember a couple of things.
Ignition on!!
You can't damage anything by using the testlight.
The testlight should come on full brightness in all of the tests except where you use it to ground the relay coil.
Use simple logic and don't jump to conclusions.
The hard bit is access - in this circuit, the easy bit is the diagnostic.
Good luck - should be back around 18:30