The pressure line switch is unreliable. Later Mogs have a mechanical switch on the pedal.
It's a simple enough job to fit a microswitch and relay if you have the earlier set up.
http://www.talkmorgan.com/ubbthreads.php/topics/31869/Traditional_Cars_Indicators_Br#Post31869


DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4