Coolant Level light is blinking

I don't believe the two are related. There is a sensor in the reservoir. Did you replace that?

The two are on seperate circuits. Chances are there is a short from the coolant level switch wiring that is causing the light to come on and off. I hate to say it but you need to start tracing wires to check for damage.

That third wire to your horn is for your alarm when it goes off that make's it honk and when you push the lock button twice that third wire sends a signal to your horn to let you the alarm is armed.I would get the correct horn so all that work's correctly you could even find a used horn. As far as your low coolant light goes first thing to check before you start checking all your wire's is make sure your back up fuse is good if it is blown that will cause your low coolant light to stay on. If that is good then unplug your coolant bottle and jump the terminal's to your low coolant switch together and start the car and give it a couple of mins and see if the light is still on. If the light is still on after all that then start tracing the wire's.

It's in the under hood fuse box it's labeled B/U LP I posted a diagram so you can see where it's located.