Your vehicle should have 16 spark plugs,(two per cylinder) and they are found at the sides of the motor below the valve cover. To replace the spark plugs it is bet to remove the engine cover, air filter and all of the coils with the ignition wires.There is a tool for removing the wires from the spark plugs and if you are not careful you can damage the ignition wires. Wire damage can cause the engine to run rough or the check engine light to flash. When installing the new spark plugs I always lube the threads of the new plugs with anti-seize or synthetic ATF. Make sure you put the wires on the correct spark plug when reassembling, it is possible to mix them up. Mixing up the wires may not affect the driveability of the vehicle, but it may affect the smog emissions. Good luck.
THE INFORMATION BELOW ("Your vehicle should have 16 plugs") is very INCORRECT. Changing the plugs on your '97 E420 is quite easy.
Pop the hood and you'll see the engine cover. Disconnect the air filter housing (it's two hand-clips). Then to remove the plastic engine cover, I believe there are two or four male TORX bolts (in the front and back). Remove those (you can use a standard metric socket). Then, look on the front of the cover. You'll see two circular little things. Gently but firmly pull those out and up, and the cover, hose, and filter housing should all come off. Set it aside. Now on each side of the engine, on top of the valve covers, you'll see the coil covers. There are FOUR per side, EIGHT total (once per cylinder). Once you get that cover off, you'll see the four coils and the wires attached to them. Because of the way Mercedes assembled the setup, you really can't mix them up (you'll see when you get in there) so don't worry. Take the wires off the coils (gently) and set it out of the way. Now, take out the coils. They come off with metric hex sockets (I think 5mm). Pull them out and you'll see the plugs. Take a spark plug socket (5/8" or 16mm) and an extension and remove the plugs. Take your new ones, put some anti-seize compound on the threads (not too much), and reinstall. DO NOT OVERTORQUE THE PLUGS!!!! You have an aluminum cylinder head and will strip the threads and cause 4-digit damage. If you have a torque wrench, torque to 11-15 ft-lbs, NO MORE. If you don't have a wrench, tell a woman to tighten them gently! Reconnect the coils, wires, and put the cover back on. Repeat on the other side. Put everything back together and enjoy your recovered power and gas mileage.