Same as above. You can set it to one on the 2 custom buttons on the steering wheel and dash. I set mine to the wheel. Go into the PCM and find the assign custom button and scroll down until you find Park Assist. I use it all the time.
Regardless of which version of Park Assist you have it can be displayed as others have mentioned - specific button on the lower display or you can program one of the two diamond keys. Note that it is speed sensitive and disabled at speeds > 12mph or so
Leaves the other 2 programmable buttons free for other things (media source on steering wheel / next track on dash) as I use those a lot and voice activation is patchy at best.