I have a short on my horn. Every time I hit horn button the fuse blows then door locks windows and insides lights go out
help would be nice.
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You probably should give us some more info. The year of the car for example. Have you done any mods to the car electrical system? Did you just get the car ( since you are new here I assume that is the case maybe) and the previous owner might have messed things up. If you just got the car you should get yourself the AIM ( assembly manual( and a wiring diagram so you can follow the circuits and all of the accessories that are on the horn circuit ( you have already found that out). The first thing I did when I got my 78 was get a laminated color schematic, 2 pages of it. It really helps understand what they were thinking when they built the car. The horn circuit goes from the button down to the a connector at the bottom of the steering column under the dash. It could d be anywhere from the button to the horns up in the front of the car. First up I would disconnect the 2 horns, then push the button. That would tell you it is the horn, or it would rule that out. Maybe find the wire for the horn at the connector, disconnect it and run a jumper to the fuse box just for the horn. If it blows, it is in the steering column somewhere. If not, it's further down the line. Just a guess because some owners really make a mess of the electrical on these old cars. Welcome to the world of C3's. I hope you are mechanically inclined!

 

Horn relay is in vastly different places on these cars throughout the years. You know the short is in the horn circuit. I would start at the horn relay and check wiring with a ohm meter looking for a short to ground. Good news is it's not likely to be in the column. The horn wire that goes up column to the horn button is a ground wire. If it where shorted you wouldn't blow fuses. Your horn would be going off.
I'd be looking at a shorted horn. Or wiring from relay to the horns shorted to ground. Also test the relay.