Should be a simple matter. An 81 should have multiple belts, one for the steering pump, alternator, A/C compressor and what ever driven accessories you may have. Some of these may be double belted. One or more will also drive the water pump.
Looking at the front of the engine, you will have to loosen the accessory driven by the frontmost belt, then work that belt over the fan. Continue to loosen items and removing belts until you get to the belt which drives the smog/air pump. Most of the accessories will be held in place by a minimum of two bolts, one of which will be a pivot point, none should require complete removal of the bolts to facilitate loosening the belt.
Chances are the smog/air pump will be the belt closest to the block. Since you've removed all the others just to get at this one, might as well replace them all. When you tighten the accesories and belts back up, you'll want to shoot for something in the area of 3/8 inch of deflection on the belt. That means, when you push on the belts, midway between pullies, the belt will move about 3/8 inch. Too much slack and the belts will slip, too tight and you could damage bearings/seals on the accessories.
That's about the best I can tell you. I know nada about the 380 or an SL, but the drive belts are a pretty generic thing.
Hope it helps.
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'02 CLK320 Cabriolet - wifey's mid-life crisis
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