Ml350 brake lights not working

If you can copy and past this to google search I think it will help.
search for third brake light replacement. It even addresses the issue of the cracked lens.
http://www.benzworld.org/forums/c209-a209-clk-class/1451406-third-brake-light-replacement-diy.html

One of these might work.
Mercedes class reset brake service light YouTube

have you checked both front and rear sensors?

Possibly the brake light switch on the brake pedal.

Did you check the fuse? Either the fuse may be out, or the brake light swith on the brake pedal may be out or need adjusting. You should have 12 volts at the brake switch.

Rick,
The brake lamps on your Mercedes are computer-controlled. I would recommend scanning your computer network for possible fault codes.

IF YOU OPEN BRAKE LINE ON BRAKE CALIPER WHILE CHANGING BRAKE PADS IT WILL CAUSE BRAKE FLUID LOST CAUSING AIR IN BRAKE SYSTEM IN WHICH BRAKE LIGHT WILL COME ON.YOU NEED TO BLEED BRAKES.

Mine kept telling me both left and right brake lights needed replacing and I noticed that on one of the bulb socket contacts that touch the metal plates on the right brake light, the plastic behind it was melted creating a crevice therefore not making good contact. I added aluminum foil behind the contact (both circled in red in the pic) and the plastic and walla, saved 50 bucks worth of parts ;-). Fixing the right brake light fixed the left one too so tackle the right one first.

check your brake fluid, if the brakes are wore that bad then the brake calibers extended out all the way which will use more brake fluid and trigger the sensor to warn you, or worst case you are leaking brake fluid somewhere.

this needs to be done by a mercedes dealership/garage