PSB 1800 LI-2 PBA (Power for all/P4A) Bosch Green drill horn adapter plate.

Bought an airhorn kit from amazon for £25.  These things seem to be fairly universal. 

https://www.amazon.co.uk/dp/B0BY2J7Z4F

Used two wagos to join the electronics.  The print is “friction fit” and just replaces the motor.

If you have one of these drills gathering dust, give it a new lease of life.

HONK!

Sticking 18v into a device which takes 12v is a recipe for disaster so don't complain if it melts or explodes killing your pet.  Also don't send it back to the seller, that's naughty. You broke it, your fault.  Seems fine to me though.  No warranty emplied if it hurts you either, this is very much at your own risk.  Also, it's loud.  Really loud.  Don't damage your or anyone elses hearing.  This goes in a car which will have a fair bit of metal insulation between you and it.  Honking this into someone's ears is downright stupid.  It's bad enough honking a distance away.