This is the definitive W202 seat problem thread:

http://www.peachparts.com/shopforum/tech-help/132934-98-c280-power-seat-problems.html

However, it is always the relay in my experience. I would question how the relay was tested. As discussed in the above thread, the circuit board in the relay develops circular cracks in the solder around some of the pins.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra