Ford horn relay - why is it reversed?

I've tried searching for an answer but come up short, hoping someone here can help this noob.

I have a 1999 Ford F250 and I'm trying to hook up an aftermarket horn (electronic amplified air horn) to the OEM horn switch on the steering wheel.  I've wired the new horn sucessfully for testing with a separate switch but I'm stumped at getting it to switch off the steering wheel.  I've located the horn relay in the fuse block but it seems to run opposite of how I know a relay to work...

The aftermarket horn needs +12v to activate it's switch, however the relay seems to always have +12v running through it's circuit and the OEM steering wheel switch interrupts this +12v when activated instead of sending +12v which is what I need it to do?? 

In other words,as soon as I hook up the aftermarket horn to the stock relay, it starts going off.  When I hit the steering wheel switch, the horn stops.  The opposite/reverse of what I'm looking for obviously.  How can I get the steering wheel switch to send +12v instead of interrupting it?

Thanks in advance for any help.