FWIW a 97 Toyota has a 22A horn relay in the fuse box under the hood. There is about 6 feet of wire from the relay to the horns. It looks like the stock horn wire is about 18 gauge which will handle 8 amps for a max of 7 feet according the the wire chart I use. The stock horns are 4A and on an amp probe pulled 2.55A. I don't know how many amps the Hellas are, but if the are less than 8A the stock wire should work. Again just make sure of the ground. The bracket the horns are now bolted to is itself bolted on both sides. If you want to be sure of the ground, run a wire to something like the battery negative, frame, or a good spot on the body. If it were mine and I decided to use the Hella relay, I would eliminate the Toyota relay. Otherwise the Hella relay is being pulled in through the Toyota relay.
Bill