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.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)