Behind the hubs of course. You can do the rears with the car on the ground and wheels on. Just put jackstands under the frame rails in the back just in case and chock the wheels. Slide under the car from the back. If you need a little more room start the car and sit in the trunk and let the RAS inflate - then flip the switch in the trunk once it pumps up, and turn off the car to keep it high.
Just a 10mm (iirc) head bolt on each side, then they slide out (from the back) and you can clean it with some brake cleaner. The wiring to the sensor looks like a tube. Can't miss it. BE CAREFUL taking the bolt out, they are prone to snapping - use a short 5 point socket to minimize 'rocking' on the head while taking it off.
The front you need to take the wheels off, but it's the same process other than that. Slide them out and spray them clean.