An 88 300SEL has the M103 engine. If you're looking at the engine from the front, the spark plugs will be on the left side... the side opposite the dipstick. You should see 6 cables loop from the valve cover (long black thing in the middle of the engine bay) to 6 points on the side of the engine. Each of those terminals hides a spark plug.

Sixto
91 300SE
87 300SDL
81 300SD