Bmw e 39 1999 air bag light comes on and seems to

SOURCE: Replacing visor courtesy mirror glass

two solutions, One www.car-parts.com or go to glass shop and see if they can repair it.

SOURCE: sun visor won't go back into place

if its flopping around or feels loose you can try tightening the "hinge" that connects it to the roof and tightening the screws on the visor itself. That might align it enough to fit again properly.

SOURCE: sun visor replacement

You unscrew the two bolts and gently take the visor off the mount. There will be a wire attached that has a plastic male/female connection that can be pulled apart. Do this gently. The wire is the power connection for the vanity light located within the visor.

SOURCE: air bag light on constant, horn not working,

you need to have the codes read for the air bag to know for sure which direction to start ,but yes, they are all tied in to the clock spring.

SOURCE: on my BMW 528i while driving radio stops working,

low power, bmw ecu has some code that starts shutting down systems on low power. so poor charging from the alternator to the battery.

this could also be a wet ecu tracking false codes.
i don't have direct experience with a series 5 but also check your ecu condition.

the ecu compartment is under the windshield on the passenger side on E36

you gain acess to it under the hood....
this compartment gets wet, the ecu lays on a flat surface in that
compartment.

the bottom side of the circuit board gets wet and starts corrosion.

remove the ecu, take the bottom plate off, clean the bottom of the circuit
board with alcohol and a soft tooth brush.

you should be able to see the signs of corrosion or conductive build up.

this gunk bridges solder points doing all sort s of wierd things to your
car, including false sensor readings from asc/abs controllers and other
sensors....

all the way to engine failure and worst case burnt and shorted wiring
harness.

I'm on my 3rd ecu, took me and the repair boys a few years to figure this
one out.

BTW: i would get that ecu mounted above that flat surface, use some sort
of standoff to raise it so that it is not laying in the wet.