I know this sound repetative and picky but how did you test the switch?
With a vom connected in ohm scale 10k connect the leads, activate the seat switch slowly and release slowly and watch for any variances while doing so. Then get on the seat and check it while physically sitting on it. Move around leaning in different directions.
The reason i seem so picky about it is because most techs just push the cushion down and if it makes they thi k it is ok when it may have something else going on with it. Seat cushions wear, the mechanism inside the seat cushion can become malformed and cause the switch to open at the slightest wrong move or just be in a way that is not pushing the switch 100%. A bad spot in the switch movement.
Get where I'm going with this?

If you have xone all tbis and ruled it out the i would start checking connections and check wiring for breaks or chaffing.