I've done a little digging for you. The location of the smart unit (which this electric warning buzzer is a part of) in the 2013-2018 RDX is the same as in the TLX and the Honda Accords. It's in the back, behind the bumper on the driver side.
According to this video, you might not even need to replace the unit. It might just be dirty. Take the buzzer out by pushing it up and sliding it out along the brackets, clean it thoroughly and make sure there is nothing in the hole in the middle, then put it back in.
Let us know how it goes.