I'm no expert, but give me a crack.
I know when the system gets low vcoltage, the "air bag" or some other idiot light will come on. You need to go to a dealer and they will "reset" the codes. (they should do that for free). If a light remains, you may need some maitenance.
Since you checked the fuses, I am puzzled there. Somehow, somewhere, something shorted. I would probably replace though fuses first and see what happends. Could be that the fuses looks good, but they may not be, then go from there.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)