Disclaimer: I should note, this is more of an advanced move rather than plug and play, so if your able, just attach your horn to where the factory horn is if you can and use that positive+negative.

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You would need something like this. (this may not fit the crown victoria's horn relay but is an example) Basically you plug your horn relay into the top and wire the cables back into the open socket. You then splice your horn into the wire that is hot with 12v when you press the horn button. You can find this out with a multi meter. If you do it this way you will keep your original horn working so you will need to disconnect your factory horn to keep it from sounding. All features that use the horn should use this relay so it will work with panic mode