It might be one of the wires going to the lights is broke. You can easily find that by crwling under the car with a light and go up and down your wires trying to find a break in it/them.
burnt out lightbulb!
It may be the reverse light switch that need replacing, which is on the transmission.
Absolutely no!!, Make sure the Mercedes Xenon lights you buy are designed to perfectly match the Mercedes car model you own. Using Mercedes Xenon lights will give you better night visibility when driving and as they last longer you need not worry about replacing them for quite some time in the future.For more information visit:http://www.convert2xenon.co.uk/42-mercedes-xenon
sometimes when the lights blow out the fuse also blows out. check the fuse!!
Even after replacing the switch it needs to be aligned with the shifter. Put the car in reverse and then have someone watch the lights as you loosen the switch and then rotate it slightly forward or backwards. once the lights come on, secure it in position.
The reverse light sensor switch on the gearbox needs replacing. The switch is normally located on top of the gearbox & around the middle of underneath the air intake box.
Try replacing the back-up light switch. It is located on the left side of the transmission. It is a common problem.
you could have possibly bought the wrong bulbs and/or fuses. or when you were replacing them you may have damaged the wires. Also the reverse light switch may be defective.
It's a matter of replacing the reverse position swith in the transmission. This switch tells the computer wheter it's in neutral, or reverse and when in reverse it triggers the reverse lights.
Where is the fuse located for the reverse lights
Check your fuses
Try replacing the headlight switch. Do the front parklights work and not the rear ones? If so check the harness to the rear of the vehicle for breakage.