Where is the battery on a 1989 560 sl?

The battery is located in the trunk, on the right side. You should find a small black knob on top of the inner panel. Unlatch the knob and the panel should drop down. You should see a big, black plastic box with two metal lock down straps. Unlatch those metal straps and remove the black cover. There's the battery. If you're going to replace the battery, I recommend a DIE HARD from Sears. They cost just a couple of dollars more, but more than make up for it with the performance and their warranty. Sears still has the best battery for the money. Also, make sure that it's the proper size....these Mercedes use a very powerful battery. Again, the DIE HARD never let me down. Also remember, a battery usually lasts for no more than 4 years in So Cal...less in colder climates.