Try flickering the hazard switch several times before you are going to exchange the relay. The hazard switch being dirty/sandy/dusty causes mostly the malfunctioning of the blinkers. Flicker it a couple of times and hope to get a positive response.
The (combi)relay is situated under the lid on top of the fuse box. You have to remove the lid after which you will be able to get to the relay (it is located between the fuses and fire wall.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)