yeah, either that or the seat belt switch (in the clicker) in the new seats could be bad or unplugged.Julienz,
I had the SRS light on in my 1999 S420 and had to replace the car seat sensor pad to clear it. It sounds like the pad in your new seat is faulty, not hooked up, or the wrong part for your car. You could look up what the part number should be for your S420, and then check it against the part number from the S600 donor. Maybe you just need to move the sensor from your old seat into the S600 seat. It's not a big job, only the seat bottom needs to removed, not the whole seat.
Clark