Finely chop vegetables and mince the garlic cloves.
Heat margarine in a saucepan. As it begins to soften and melt, add the vegetables and garlic, cooking until soft. Add the Marsala wine, and finely chopped parsley. Allow vegetables to cool. Add mayonnaise and seasonings. Mix in Ritz crackers, crushing them as you go.
Stuffing should be firm and hold itself together. If still loose, add some breadcrumbs.
Stuff caps and bake in 350F oven in a greased pan for 25 minutes or until the base of the mushroom is soft.