The information on this thread described the exact problem and solution to a problem I've been having with my C230 for years. Can't say how many times my wife and I have driven with the seats adjusted wrong while waiting for the adjustment controls to start working again over the past 5 or more years. My C230 had only one large black relay in the forward right corner of the trunk under the large black screw down cover. The relay lays on it side and connects directly into a connector on the wiring harness. The whole repair probably to 30 minutes. Here are the steps:

1. Remove trunk carpet.
2. Unscrew 3 large plastic screws in large black plastic cover panel that covers the entire forward trunk floor. I was able to unscrew with my fingers.
3. Remove the cover to reveal electrical components including the seat adjust relay.
4. Use a small screwdriver to release the plastic retention hooks while wiggling the relay out of the connector (pull toward front of car) to remove relay unit.
5. Use a small jewler's screw driver release the cover from the relay.
6. Examine the solder joints on the circuit board with a magnifying glass. Mine had the exact circular fracture on the large solder joint near the middle as show in a photo in this thread.
7. Use a soldering iron to reflow/apply more solder.
8. Reassemble.

Seat adjustment works great. I'm glad I declined my mechanic's proposal to replace the computer module a while back, a repair that would have cost over $1,000!