When I purchased my 1985 Mercedes 300D, the windshield wiper intermittent setting (detent 1 on the wiper selection bar) did not work properly. When selected, the windshield wipers would either work exactly like the slow and continuous setting (detent 2) or they would be intermittent for a cycle or two and then go back to slow and continuous. After some googling, I found out that a relay is responsible for turning the wipers on and holding them off for the short delay and was the source of my problem.
To purchase a new (used) relay on ebay costs approximately $30. This seemed too much for me and I don't know of any good junkyards around me, so I decided to take the relay out, open it up and attempt to fix it. After only a little bit of work and some light soldering, I was able to fix the relay and now it works just like new.
Step 1: What You Will Need
No. 2 Phillips Screwdriver
Vacuum Desoldering Tool (Not strictly necessary, but incredibly helpful)
47 uF/16V Electrolytic Capacitor (QTY: 2)
*Note: These can be rated for higher than 16V. The originals are 16V, but a higher voltage rating (25V, 35V) is fine. However, these MUST BE 47uF so that the timer delay (RC Timer) in the timer circuit is preserved. These can be purchased at RadioShack.
Something that can give you at least 12V (2 9V batteries, One 12V Battery, 12V Power Supply, etc.)
Step 2: STEP 1: Remove the Relay
Now, use your flashlight to light up inside the dash where the kick panel used to be. The relay is hidden behind a heater tube against the back wall behind the instrument cluster. If you were to see through the instrument cluster, it would be almost directly behind the fuel gage. It will take a little bit of stretching but should pop right out.
Step 3: STEP 2: Open up the Relay
Step 4: STEP 3: Test the Relay itself
If the relay does not switch, then the relay itself is bad and it may make the most sense at this point to go out and purchase a new Bosch part. If it does switch, move on to the next step.
Step 5: STEP 4: Check out the Electrolytic Capacitors
Note the location of the positive and negative terminals on the capacitors. These are important. The second photo is a slightly more clear sketch showing the location of these terminals. On newer capacitors, the positive side is generally not identified, but the negative side is indicated with a lighter colored stripe down the side of the capacitor.
Step 6: STEP 5: Remove the Electrolytic Capacitors
The second photo shows the relay module with the capacitors removed.