low beam flashes and then wont come on.

Based on the information provided, it appears that you're facing an issue with the low beam on your 2003 Mercedes-Benz SL500. Since you've already replaced the xenon HID light bulb, and the problem persists, here are some possible causes and solutions: 1. Faulty ballast: The HID system relies on a ballast to provide the required voltage to ignite the bulb. If the ballast is faulty, it can cause the low beam to flicker or not work at all. Consider checking and replacing the ballast if necessary. 2. Wiring issue: Check the wiring harness and connections for any signs of damage, corrosion, or loose connections. Faulty wiring can lead to intermittent lighting problems. 3. Fuse problem: Inspect the fuse related to the low beam circuit to see if it's blown. If it is, replace it with a new one of the correct amperage. 4. Control module malfunction: The headlight control module could be malfunctioning, leading to the issue with the low beam. A diagnostic scan may be needed to identify this problem. Given the duration of the problem (6 months), it's essential to address it promptly to ensure your safety and visibility on the road. I recommend having a professional mechanic inspect your vehicle to diagnose the exact cause of the issue and carry out the necessary repairs. They will use specialized tools and expertise to pinpoint the problem accurately. Feel free to provide more details or ask any specific questions if needed.