I replaced the plugs in my '94 S420 and it was a smooth and easy operation. You simply remove the air cleaner and center protection plastic pieces and the plugs are right there. They are deep but with the proper spark-plug socket and extension, easy to remove and replace. The spark-plug wires, likewise are in easy reach (though I didn't feel the need to replace them). I'd say my '94 S420 (140 body V-8) was one of the easier cars to change plugs on (ending in a preposition).