It is, as suggested, a RF interference suppressor - a capaciotor across the power to the wiper, I suspect, to suppress interference from sparking brushes. 
It may have been fitted to GS models because it (or even just its bulkhead) could have a past history of being a FFR, or simply because it had been found that a GS close to a FFR was interfering. (same way that all of them will have suppressive ignition leads, along with all petrol cars since the introduction of TV to Australia in 1956)
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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