Foul smell in fridge?

I have a terrible smell in my fridge that I don't seem to b able to get rid of. I ahve emptied it and washed all shelves and the interior of the fridge but still the smell persists. It seems to be coming throught the vent from the freezer compartment in through the top of the fridge. The freezer compartment does not smell nearly so bad as the fridge. I think a mouse or something may have got trapped and died in some vent or something so wonder if you can advise how I access the vent that circulates the cold air as the smell seems to be circulating through the vent in the top of the fridge. Make of fridge/freezer is Amana, TS518SW