jetschin,
Sorry to hear about your trouble.
You seemed to have 2 problems:
1) Water
-Water can come into the space directly below your E39's passenger seat from the ECU compartment, if not remedied, your ECUs will be fried and replacement is in the thousands of course.
-Water can come in from the windshield, spray water on windshield continuously and check.
-Water can come in from the door, spray water and check.
I suggest you open the hood, take out the black plastic cover for the filter(the one nearest the windshield, in front of the passenger seat), your ECUs are below this space.
Take an airgun and shoot air into the space, see if you hear water. If you hear water, then quickly remove whatever causing the clog, remove the passenger seat, remove the satellite sensor, and try drying everything.
If there is water, there is a high likelihood that something has clogged the passage, and water is building up, which will seep into your passenger compartment.
This water will then stay directly underneath your seat and starts to build up, and eventually fry the passenger seat air-bag satellite (RM1000+).
If you notice your air bag sensor goes on and off, you better stop driving, and inspect the sensor, before it gets fried. You can take out the entire seat, remove the satellite and dry it (depends on your luck).
2) Air bag sensor light
-There are 3 major problems
1)tensioner-usually can be fixed by experienced 5-series technician, just need to tie down the cables to make sure they don't follow the seat movement (e.g. moving the seat front & back).
2)seat mat-most likely due to wear & tear or putting heavy material on seat, can try bending back in shape but most likely needs replacement.
3)satellite-most likely due to water underneath seat, locate source of water seepage.
Good luck. In case you need further help, PM me.
