Says here:-
http://www.bobmckay.co.uk/viewtofront.html that there only needs to be sufficient fluid to be able to wash the view to the front and given that there is no requirement to have a rear wash/wipe, that's why I find it curious that you've been told that it's an MOT fail, front yes, rear? I'm doubtful but hey, I have been wrong plenty of times
As for the wiring, haven't looked at the circuit diagram for that bit but I would not expect it to be powered via a relay (now watch me be wrong on that one ) You must have missed my post elsewhere about my dislike of car electrics
I'm an electronics BSc, but I like digital electronics most, two states, now it works/now it doesn't
David.