I just diagnosed mine today using the electronic service manual. I printed out page 161 which has the electrical schematic for the system. I wanted to test the voltages at the N13 (Interval Wiper Relay) first. At first I could not find it. I referred back to the manual and looked at the pictures where it should be. It shows it on the left side of the hatch to the left of the locking mechanism. Turns out, I had no relay; it was removed. I will need to start by spending the $80+ for a new relay.
Will keep all posted!

1987 300D 186Km
1987 300TD 196Km
2001 CLK 430 Convertible 32Km