One of the worst things you can do is spend a lot of money on a vehicle and Euro or black tail light guards, and leave them unprotected. You wash, then wax your vehicle, you vacuum, then clean your interior. But, then you leave the tail lights and head lights to fade or become dull.
The best way to protect your tail lights or tail light guards is to put a coat of wax on them every time you wax your vehicle. You should wax your vehicle at least once a month. This will help with scratching, fading and will keep the elements from destroying the tail light or tail light guards' finish.
Never use a dish washing soap or detergent of any kind on your vehicle or the tail lights or guards.