On my unit, if I was going to wire it in, I have lots of room behind my control panel where my switches are located. So that is where I would install it.
So I would leave the rocker switch (master shutoff). To mount the unit I would drill a small hole through the wall to fit the shaft of the knob. Then cut some scrap wood to fit between the paneling and the base plate where the mounting screws are located. Glue the two boards in place. Drill and screw the dimmer in place. Then to wire it up, I would disconnect the output wire from the rocker switch (use a meter to determine the power feed). I would take that wire and connect it to the dimmer. You might need to look at the wiring diagram for the dimmer to see if there is a particular terminal that is needed to be used. Then I would add a jumper wire between the rocker switch, where the wire was removed from to the dimmer. Presto you have it installed. Here is a super simplified picture of the wiring.