where do I find a hot-when-on under hood connection

If you just use a dash switch it will melt you need to wire in a relay to carry the load of the air pump.I would just wire it to the battery with the correct fuse size.Also use the horn wiring for the relay trigger to make the horn go off.

The air horn kit includes a relay & a connection to the factory horn as a 'trigger'. I wish to cicumvent the extra switch inside the cab by using a hot-when-on connection under the hood on the passenger side.
Better yet a ground-when-on connection ( doubtful that exists).
I tried a gnd to the provided relay which allows all horns to work together, but of course this would put a drain on the battery with the key off.
I would have provided an attachment indicating the wiring diagram, but the forum system doesn't allow for attachments.

you need to follow the instructions in the kit. when you do, you will realize that there will be no drain when it is off. it is a simple set up and is reliable. give it a chance

Roy

As I stated the instructions call for a switch inside the cab which would be a big inconvenience (adding an additional horn button).
If there is a place to access the existing horn button wiring under the dash I would like to know where.

I posted a wire diagram for the horn circuit that should help you out.