No fix. If it's just losing a little you can keep an eye on it and keep driving. Sometimes they seal up for a while. They leak 'cause the mounts for the radiator are on the side of the tanks and after a while the bumps and road vibration do the gasket in. Replacement is no big deal. Radiator drops out the bottom. After market is cheap but usually quite heavy as they are copper (I use 'em anyway)instead of aluminum. Take your pick but make sure you get the correct turbo or non-turbo one.