Locations of X35d diesel exhaust NOx Sensor Control Modules?

I bought a pair of NOx sensors. Locations of the sensors in front of and behind the cat are easy to see. But each have control modules at the end of the wiring that are not visible. About 2-3 feet long from the sensor to the control modules and wherever their mounting locations are

Can anyone help to show diagrams or photos of where these are located so I can plan on how much time to account for to make the replacements?