don't replace 'em fix 'em. Take them off, remove all electrical components and fill them with water; rotate them around and find where water comes out. The factory shells have vent holes but have a rubber 90° boot that goes over the hole to restrict water from entering. It may be as simple as adding a similar boot to any vent holes (or a simple length of rubber hose). If they are assembled from two pieces that are screwed together, disassemble put in a bead of clear silicone and reassemble. (and also work on the venting issue mentioned above).