Your best option is to find the buzzer that goes in the BCM module.

Then solder a switch in between one of the lines that goes into it:
Kia Sportage: Rear Parking Assist System Control Unit: Components and Components Location - Rear Parking Assist System - Body Electrical System - Kia Sportage SL Service & Repair Manual

Then you can cut of the connection to the buzzer, effectively giving you a "Off Button Swtich"

 

Just a quick question! Why are there no switch in the first place? Is it an aftermarket solution?

 

Not sure why there isn't a button. The tow bar is a Kia factory tow bar.

 

Is it possible to add a lower crash pad assembly that has the p-off button? Can I just swap over the assemblies, plug the existing cable into the new assembly and hope the button works?

 

I have a 2011 EX with factory backup sensors. There is a switch to turn them on and off next to the dimmer switch...