The basic problem is that when the circuit board was made they used less than the required amount of flux at the solder joints and then used a process called "wave soldering" to apply the solder (the circuit boards float across a pool of liquid solder like corn flakes atop milk on a cereal bowl.

This flux (the goo that makes the solder flow into the joint) appears white on the circuit board at the solder joint, a tell tale indicator is small cracks across the surface of the solder joint. Use a magnifier to enlarge your close view and you will see them. This is the major cause of relay and module failure in 90% of the relays and control modules on the market. (Hella, Bosch, etc ALL are "Hecho in Mexico" or China where QA is a theme ride but not reality).

Use the cheapest Radio Shack iron and don;t add any solder, just touch the tip to the bad joint for a second until the surface gets shiny or flows and STOP and let it cool and set and that should do it.

Believe me, MB ain;t the worst- 99% of all the "no start" conditions on Volvo, Audi and VW are caused by relays or control modules where the circuit board was wave soldered.