My first guess is it's the contacts, not the mudguard. How do you connect the rear light to the conductive strip; a screw goes into the plastic and cuts into the aluminum, or something like that? If so, maybe the screw and/or the aluminum have oxidized a bit... some kind of gentle acid might remove the oxidation enough to reestabish contact. There are conductive glues, which might help... if the glue is waterproof, that is.