You may have a leak in your air springs. By parking on an incline, it shifts the load on the springs, causing the spring to collapse (or extended depending if it's pointed up or down hill) the crack on the air spring that was leaking while your car was level will be sealed up when it's on an incline. The warning light may be on for the compressor running too long. Air springs on Town Cars are a very common failure point. If it's not addressed soon, it will cause air suspension compressor to fail as well! Always replace the air springs in pairs and have someone check the codes in the air suspension module to make sure that leaking air springs are the only problem. If you are satisfied with my answer, please accept.