I have 1991 240 sedan that dies when I turn on my headlights. Also my SRS failure warning light flashes and my right blinker is blinking twice as fast, indicating it is also failing.
I recently changed the alternator, starter and battery. I also replaced the mc and right after that repair the car started having these symptoms.
Any help would be appreciated. Thanks.

 

You have a voltage problem, it's too low...
Curious why would you change all three? They couldn't have all been bad?
The MC (master cylinder)? could not be involved unless you knocked a wire loose or something like that, around the alternator... Battery connections/grounds may be loose--it could be that simple--check all those first.
The right blinker is not related to your other issue and it's coincidental.

 

I'm betting bad grounds. My brick started cutting out briefly when I switched the high/low beams.

Cleaned up the grounds to the headlamps and it fixed it. On these old bricks IMO it's a good idea to chase all the grounds and clean them.