On the switch mech there are two white tabs which ratchet in one direction of the steering wheel, and catch in the other direction, cancelling the indicator stalk switch. Only one of these tabs is presented when the indicator is on left, or right. 
There's some funky stuff happening inside the switch (springs, ball detents, brass roller contacts etc) so it can be pulled apart and serviced to some extent, but the switch proper may be the only part that is commercially available. 
I'd start by removing the steering wheel and gaining access to the blinker mechanism, and looking at the white tabs, ensuring they are not broken or missing. From there you may be able to work out your options. Worst case, it's a 70 dollar part and about an hours labour (free if you DIY!)
I'm just going off an autopsy of a switch I have here, so not 100% sure how it interfaces with the steering wheel.
				
			 
			
		 
			
				
			
			
				-Mitch
'El Burro' 2012 Defender 90.
			
			
		 
	
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