Easiest way to run a new wire through firewall?

I want to add a new horn but to use independately. I need a wire to go from a new horn button to the engine compartment. Any ideas?
I once had a Camry that the weak horn operated normally unless I also had my foot on the brake petal. Then, the loud horn blew. I could select loud or soft.

 

Thank you. This is exactly what I was looking for.
At first, I was a little confused by ebay commercials in between posts. The commercials looked highly technical
but did not make sense.

 

That is how I did it on my Camry and plan to do on my Prius. But I still need to run a wire through the firewall. I doubt that the existing brake light wire goes in to the engine compartment.

 

No. Every time the horn blows, the brakes are applied.
I connected one side of the new relay coil to the brake circuit (normally zero volts). I connected the other side to the anode of a 1 amp doide anode. The cathode connects to the horn button circuit. x

 

Chap- I was answering a prior post. I am trying to learn how to include the prior post with my answer.

 

I have to add a relay and a diode for it to work correctly.

 

No. I was posting a (funny) reply to a post that asked if the horn will blow when the brakes are applied. I thought that the person was joking. I apologize.
So, here is what I think will work. see attached drawing. The diode leads must be secured so that they do not flex due to vibration. I think the hardest part is running a wire through the rubber grommet in the firewall. It is hard to reach.

 

It did strike me as funny if the horn would blow when touching the brake petal.