STOP... DO NOT FOLLOW THESE PROCEDURES (yet). DO NOT BUY A NEW SENSOR YET.
THE SOLUTION MAY BE A LOT SIMPLER, as already indicated by a previous poster.
MB are great cars mechanically, but electrically they are behind a bit. (not sure if they lack incentives to improve).
As indicated before a few times on this thread, one of the 3 sensor wires between the connector and the sensor box under the seat cushion can break. I had the same problem. Unfortunately I had already bought the new sensor (as the dealer told me it had failed and that it was very common. SURE).
So what most likely is going on is the following:
1. Each wire has only one thick thick and brittle strand of copper.
2. The way the tie wraps are placed under the seat to hold the cables together it seems that stress is placed on the wires as the seat is moved up and down and you know what happens on cheap single strand wires when you flex them back and forth a bit. It only takes one and only one strand crack for the circuit to open.
In my case the white wire lost continuity between the yellow connector and just as it went into the hole under the seat. Most likely the crack would occur near the place where the tie wrap that ties it to under the seat.
So here is what I suggest:
1. Once you are sure that the SRS alarm is caused by the seat sensor failure, remove the passenger front seat from the car, following the instructions at the beginning of this thread. (I disconnected the battery just for insurance)
2. Test the continuity for each of the 3 sensor wires from the connector to a place near the hole where the cable with those wires goes into the seat sensor box (not visible at this point). As I said before, if you were to be lucky to have a broken wire it will most likely be near the places where tie wraps attach the cable to under the seat.
3. If you find a broken wire then you have 2 options. Either bypass the broken wire using safe and proper procedures using multi strand wires with the same kind of insulation as the originals (note that this is not a trivial job for an amateur). Or replace the whole sensor anyways so as not to have patched wire works under the seat.
I chose to replace the sensor as I already had bought a new one. Yeah yeah I could have saved that $100, but I wanted to have all I could need when I decided to take the thing apart and the experts had already told me that the sensor was bad.
Incidentally, removing the leather cover from the cushion requires more than 1 person so that while one compresses the seat the other removes the bottom bar that attaches the leather to the base. The photo on this thread does show 2 guys doing that step and you need it so as not to damage the leather using tools to force your way. I was able to remove the leather without any tools or screw drivers with 2 people.
Do not remove the screw on the back of the seat, as indicated on the thread. The picture shows the screws all removed and the seat cushion completely off from the main assembly. I know why that was done, as connecting the clips to the wires on the leather is a tedious process , but if you follow the initial procedures using longscrew drivers to re-attach the leather wires to the clips you will have to disconnect a bunch of wires, including the seat belt clip sensor wires. I have seen many people who contributed with ideas on how to connect the clips to the wires, but they all require some trial and error. An idea just came to mind (maybe the thread starter did this way but did not provide details enough), if you were to flip up the bottom of the leather upwards you should then be able to see the wires and the clips and working from the back to the front hook them all. The leather is very tight at the bottom ends so this maybe difficult and the fishing method maybe the only one, but I find it hard to believe it was the way the seats were put together the first time.
The real tedious part is to have the seat assembly on the floor (use a carpet so that you don't scratch solid floors) and to try to work on the seat cushion while bending down. Ideally you should do this work with the seat assembly on top of a workbench or a table with some cover to protect it. That way you don't need to keep bending and can see the work better.
IMPORTANT: When routing the cables under the seat keep in mind that it moves up and down as well as forward and back so leave enough slack when using cable ties so that they don't get stretched or bent too hard, which would cause another failure very fast.
If you decide to do this work be patient , make sure you have the correct size star bits. Get someone to help removing the leather and have a beer at the end as you would have saved >400 in charges. If you take 4 hours to do this job then consider yourself a highly paid part time worker on a day off. Not bad. If you get your son to help you then make it one of those quality time father-son bonding moments we seem to have very little nowadays.
Now I am expert in fixing yet another MB problem I never had with any car in the past. Mind you I did not have many with electrically adjustable seats. LOL, but they did have MAF, Crank position sensors, etc and until I bought a MB I had problems with those before.