with the engine off, use a GOOD multimeter and measure across the battery terminals.
Write this down ( ex : 12.33 volts )
Now measure from the positive terminal to any good contact area on the engine.
You should see very little difference.
Even if the readings are the same, leave it hooked up this way and wiggle the heck out of the negative battery cable towards the fender frame ground point.
If the voltage then changes, clean up the ground connection or replace the negative cable.

Even if you DO decide to replace the PTCM, go to car-part.com and see if you can find one.
I'm pretty sure they are the same in the 2001-2004 SLK320, someone correct me if I'm wrong.
I also do not think these need any coding, just plug and play.