Are you talking about the rear work lights (add-on) or the OEM hazard/turn signal lights? If you mean the OEM hazard/turn signal lights, replacing those could get pricey. You may try drilling a small hole in the bottom of the fixture for water to get out, and seal the top with a bit of silicone caulk.
When I bought my used tractor, my lights were filled with water from the techs using a pressure washer to clean it. I drained them, used dielectric grease to prevent more corrosion, replaced the bulbs with LEDs, and parked it under a cover when not in use.