Bad motor mounts will break your air cleaner mount. I made some notes when I replaced the motor mounts in my 82 300d.
It has been some time since I did this, coments welcome.

The 300d has 3 mounts. 2 located on each side slightly back from the front of the engine and a single rear mount at the aft end of the transmission.

Front motor mount replacement:

Each mount is secured from the bottom by a single 10 mm threaded allen head bolt (8mm head) located in a recess.

Removal and installation:
Raise the car for access. From under the car, remove the allen head bolt with a 8mm wrench. Using a jack, lift that side of the engine several inches. That will raise the mount arm off of the mount. From under the hood, remove the mount shield by working it out. Note the orientation of the shield. If the shield is hard to move, lift the engine with the jack a little more. Once the shield is removed it will allow access to 2 small allen head bolts that retain the mount. Remove these with a 4mm wrench. Remove the mount, noting the orientation and replace it with a new one. Reinstall the small allen bolts and the shield. Lower the jack, being sure the shield is aligned. From the bottom of the car, reinstall the allen bolt (t may be necessary to lever the engine slightly to align the holes.) Repeat the process for the other side.



Rear motor (transmission) mount replacement:

The single rear motor mount is located at the aft end of the transmission. It is supported by a brace that is held by 4 bolts (17mm heads) to the chassis. The brace has 2 bolts (13mm) heads that attach the mount to the brace at the bottom.

Removal:
To replace, lift the car for access and support the transmission at the aft end with a jack. Loosen the 4 (17mm) bolts and the 2 (13mm) bolts slightly, then remove the 2 (13mm) bolts that attach the support to the mount. Then remove the 4 (17mm) bolts, and remove the brace. Lower the transmission slightly (careful not to lower too much) to access the 1 (19mm) nut that retains the top of the mount. A 19mm box end wrench will work well here. Remove the nut and the mount will fall off.

Installation:
Install the mount using the 19mm nut at the top first and tighten securely paying attention to the alignment. Raise the transmission slightly with the jack. Start all of the bolts holding the brace to the chassis and the mount to the brace then tighten securely.

82 300d 194k