Look at the 4-way flasher switch. After using the flashers, the switch does not always move the whole travel in the off position, even though the flashers are off, and this short travel disables the turn signals.

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1991 300 SEL
218,000 Km

"Xsbank's rules of mechanical intervention: Always go for the easiest solution first; 90% of what ails a gasoline engine is ignition; After that its all a WAG."