Car is 1999 E430, 110,000 miles

(i) The SRS is always on EXCEPT for when a passenger is seated. Does that not mean it has to be the seat sensor that is bad?

(ii) Ohter threads say that after fixing whatever is causing the SRS to show a fault it will still need to be reset. However when a passenger is in the car the lamp doesn't come on. So if the lamp does not come on is there still something to be reset?

I'm trying to figure out if I should spend the $150 to replace the seat sensor and see if that fixes things or do the scanner ($90) first.

Thanks in advance,

Peter