Start with a couple leads of wire from the pos and neg posts of the battery, disconnect the horns and touch your leads from the battery to the horn leads. If the each horn works, you know your problem is elsewhere if the horns don't work when direct power is applied you've found the problem.
As soon as I hit send, another possibility came to mind. Worn slip rings on the steering wheel can also cause a no horn situation. About $75.00 through PartsShop/FastLane, and not too difficult to do (but does require patience). If the horns work under direct power, you can then remove the steering wheel and jump the two horn contacts (they should be the two outer contacts). If the horn works there, then your problem is the slip rings.
Hope this helps.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.