Okay, so I've tapped into the gray wire at the bottom of the light switch... only the right parking lights flash, though, which I thought might happen. It looks like the gray/white wire next to it is for the left parking lights, so I will wire in a relay to get them both flashing.

Do you have any idea which wire goes to the horn?

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)