I originally replied to this because I have a Bosch DW, and it's never frozen, even though the kitchen wall is old with the original (sketchy) blown in rock wool in a 1906 wall with no sheathing, so it's a ridiculously cold wall by modern standards. It is also a north-facing orientation.
I had the great luck of installing a hydronic toekick heater underneath the sink cabinet. This heater apparently keeps all the pipes warm when they need to be. This is nothing I planned, but it has served me extremely well these past 2 abnormally cold winters.
My Bosch has a matching wood toe-kick attached to the metal one with velcro. So there is even less room-air circulation than there would normally be
I got to thinking about this later. The water line actually does go along the floor, that is the part of the water line that is built into the dishwasher, and the pump is located under the dishwasher, close to the floor. The problem might not be the wall, it might be the floor and the ice maybe forming in the pump itself.
If there is any clearance under the dishwasher, slipping a foam panel under the dishwasher might solve the problem completely, even a thin foam panel might do it.