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I see three possible problems with your diagram
first the horn switch is to ground, you have it on the hot side.
second the added switch is momentary meaning you will have to hold down both the horn button and the switch to get the horn to work.
and third and most important the second relay has no ground. so the new switch will do nothing. I also believe you were trying to use the cig lighter to power the new horn so you will not burn out the fuse for the oem horn.
Use a single pole double through switch to change the output of the horn relay from oem horn and the new horn relay. Use a single pole relay to supply power from the cig lighter or directly through a fuse from the battery to the new horn.
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