WE HAVE TOTALLY FIXED OUR OWN RELAY!!!!
We actually have three 1999 C280's and two of them have intermittent problems with the seats working. I did a little research on line and discovered that we could fix this ourselves.
We removed the carpet, opened the 2 x3 foot plastic cover. You just turn the three screws and lift off the cover.
Under the cover, up towards the upper right hand side is a relay. It's a 1x1x1.5 black relay. The number is 002-542-22-19. I remove the module by pulling back toward the back seat, noting that the top of the case had raised writing on it.
We opened the case by prying it open with two knifes. We (husband and me)
soldered the contact points on the board. We could see a couple of very fine cracks at the solder joints and guessed that was the trouble.
We put it all back together and the seats work like new!!!!
We could have just replaced the relay but we would have had to wait for several days to get a new one so the repair was faster. The relay is available at Amazon.com for about $37.00. 3 2014