I have a 99' C280 and found your forum while attempting to fix a power seat problem.
I had determined the seat relay was the problem and even bought the new part, but couldn't figure out where in the car the relay was hidden. Many posts talked about it being in the trunk, but looking in the area that holds the spare time and fuse box I could not find it (the one that is in that area looks the same but is a relay for the suspension and air). One of the great people in this forum posted a picture of the location and looking at the picture I immediately knew that the portion of the floor of the trunk toward the front of the car ahead of the spare tire compartment was where I needed to look. Found it, replaced the part and now life is once again good.
Thanks for the help!