I have been having some or the other problem with this car but this morning, I removed the Head Unit to see if I can replace it with an after market unit. Anyway, I promptly concluded I can't and put everything back.
However, when I now try to drive the car, it gives me a "Brake" light red colored in the instrument panel and constantly beeps when I try to drive it. Is it a brake switch issue or something to do with putting the head unit back properly? Is this simply a coincidence? Please help!!
Forgot add that the HU was simply not put back. I had one of those "ISimple" devices hooked up to the back of the HU to give me a 3.5mm audio out to my MP3 player. That works fine and that was the only mod done. Did that screw this up?

It is a coincidence. Check the brake fluid level. If the level is fine, make sure the floater is not stuck.
Why have you concluded , that you cant replace the Head Unit with a aftermarket one?

I checked the floater and it seemed fine and so is the fluid. I even unplugged the electrical contacts but the light and the beep is still there.

Art, the installer for the HU told me that the speaker output was controlled by the fiber optic cable. When he disconnected the fiber optic cable, the sound stopped coming out from the speakers.

Open the brake fluid container. Start the car & press the floater with your finger. Dose the light go off? Check the electrical connector for shorts.

The amp is controlled by FO. The speakers use normal, old fashioned cables. Speakers have nothing to do with FO. When the installer pulled the FO, he "switched off the signal" to the amp, hence no sound. I have aftermarket unit installed, so do thousands of ML owners.

Dumb question. The floater is on the inside of the cap right? Also I unplugged the wires but the light is still on

I thought you said you checked the floater.....
The floater is under reservoir cap. It is attached to the cap. It will go up/down when you move it. The light will be on if the electrical connector is unplugged.

Thanks for your help Art. Yeah I did check the floater and I was able to push it in slightly. I can't tell if it's stuck out not since I see some movement. I asked again since I wanted to make sure. Also I thought if I removed the two electrical connectors then the light would go off.

Art, the reason Nick's brake light is going off is because he's driving with his parking brake on, or at least his ML thinks it's on.

Of course, he could have found this out by referring to his owner's manual.

Yeah Art I needed new brakes and rotors. NOT!! I just got those new three months ago but I appreciate your sarcasm and condescending tone. Ooh yeah and post editing, is also driving with my parking breaker on. NOT!!!
Actually it had to do with the installer not snapping something back in place below the instrument panel. It's all good now.
Thanks Art for being so helpful.

Glad you sorted it out.

That's a good one......

Yeah, I edited my post because I was wrong. Because I hadn't Referred To the F****in' Manual.

Yellow light with beep is brake pad warning. Red light with beep is parking brake warning.

It's OK bro. We all have our days good and bad. I was having a bad day and answered sarcasm with my own sarcasm. It's all good now. No brake issues.