I finally got around to replacing my coolant in my 99 ML 320. I could not see the drain plug through any opening in the bumper. The one farthest to the passenger side is blocked by some hardware that looks like it secures the plastic bumper to the car. The drain is directly behind that opening. You can see past the tow hook in the second opening in the bumper, but the drain is no where near that. Here is what I see when I look:
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So took off the right tire and I could see the drain from the rear right side. I could barely get my fingers on it. I worked with some long needle nose pliers and a small pair of vice grips to turn the drain CCW from the side. I believe I had to turn it a full 360 degrees before I could pull it out.
Here is a shot of the drain from the right front tire well:
It took 10 or 15 minutes to drain. Closed the drain with the same tools. Then I added 7 quarts Zerex G05 and 2 quarts water through the overflow tank on the firewall while running the engine. This took another 15 or 20 minutes. I squeezed the upper radiator hose occasionally to try to get the air to rise up into the tank.
I was expecting to be able to add more water, but evidently I left close to 3 quarts in there since I didn't take out the block drains.
Thanks for everyone's help.