If you just want the diagrams, i would contact mercedes (1 800 FOR MERCedes) during "regular business hours" ( I dunno, middle of the day?) and ask to speak to someone regarding technical literature. The diagrams they refer to as "ETM's" (electrical troubleshooting manuals) but are actually just diagrams. Your SLK is referred to by it's model number, which is a 170. So ask how much the ETM's are for a 170. To go beyond that to the DM's (diagnostic manuals) is a great expense, stuff is spread out for different models, so you end up with volumes and volumes and only a fraction of it pertains to the 170.
The problem you may run into with electrical diagrams is many of the wires go to control modules, and don't really explain functions of all the wires. But it would of course help in certain circumstances, like checking this switch you want to check and for checking just the wires. I bet it will be over a couple hundred bucks to get the ETM's for your car though, there are 2 or 3 volumes.