RE: 1989 300E Turn Signals
Sounds like either your flasher unit has failed and must be replaced or a bulb is burned out somewhere in the system. You might have to remove all your bulbs to check them one at a time.
Considering the year and body style you might have a cracked tail-light lens that is letting water get into your bulb area. Check this and clean up all the contacts in the rear light assembly even if they do not look bad.
I is also important to use ONLY German blubs which you can get at your Mercedes dealer. I have heard this is not true, but based on what I have seen it is.