is your fridge set to cold?
if the water on the back of the fridge never gets to melt and run down into the hole then your fridge is going to get wet and moist inside.
if you fridge doesn't have a proper temp gauge then you need a thermometer like one of those meat testing ones and a bottle of cold water.
leave the water in the fridge until it reachs the natural temperature of your fridge.
then test how cold the water is with the temp probe, you want it to be around 2-3c (anything below 4.4 but not to cold your fridge just freezes over and the back never gets hot enough to melt the moisture of the back and the condensation just keeps building up quicker than your fridge can get rid of it)
btw your fridge will work like this.
behind the backwall on the inside of your fridge will be the cooling coils which is how the backwall collects the moist air.
the cooling coils are wrapped by a heating element which is supposed to hear the backwall until it detects the temperature is 0c.
the frosty wall is supposed to melt and run down into a hole and out the back into a collecting plate above your compressor where it gets hot and evaporates when the fridge is doing a cooling cycling.
if it's to cold I think to much moisture setlles on the backplate and the heating element turns off before it's melted enough
Btw are you certain the fridge is roughly level? if it's tilted slightly forward maybe water is dropping off the ceiling and onto the shelf or the air isn't circulating very well.
infact some of the cheaper fridges don't have a circulation fan at the top whilst the more expensive models tend to.
maybe the ones without a fan are just not a very good design but if its level and not set to cold then you should be fine.
if you were to put mushrooms or something in the salad compartment would they start to frost over? if so your fridge is def to cold.