I recently replaced the left motor mount on my '99 ml430 w/144,000 miles. It was not as hard as many warned. Tools: 19 mm offset box for top nut, 16 mm deep socket for bottom two nuts, 12 mm reverse torx for engine side plate, 10 mm socket for inner fender, small heat shield and lower hose to ps pump.

Remove tire and six nylon 10mm nuts holding the inner fender panel, remove 3 10mm nuts holding small shield, remove top 19mm nut. Loosen right side mount bottom 16mm bolts, don't remove nuts. Remove two long 10mm bolts holding hose bracket for ps hoses behind pump so you can get the socket on to the 16mm bottom nuts. Remove lower 16mm nuts from left mount. Place floor jack under engine and raise the engine enough to remove strain on mounts. You cannot remove the mount without removing the side plate because of the length of the three mounting studs. You can remove the side engine plate that attaches to the mount with the 12mm reverse torx sockets (hint: a new 10mm 12 point box wrench works well enough if you don't have the reverse torx sockets as the side bolts are not torqued very tight. Don't forget the long torx bolt holding the ps pump to the bracket. Everything now falls out into your hands. This method eliminates the requirement of disconnecting the ps pump or the serpentine belt!

Dealer wanted $660, I got a mount for $33. Took almost two hours but wil only take half that next time.

 

Maybe you should have just taken the time to replace the right motor mount as well for good balance. You will be needing to do it soon anyway. Just my thought.