Carryboy Rear Wipers

I have a Carryboy in the back of the D-Max Pick-up, All good, but after a bit of dust on the car, and the back window, then when I go through rain, the back window is a total mess. I can't see out the back window through the rear vision mirror, it is not 'fog' as both de-misters work on the pick up and the Carryboy.

I want to have a single wiper on the rear window, like a normal "western" station wagon or van would have installed as standard. My Carryboy dealer is a shonky asshol_e who hates non Thais and has a biz practice or extortion and thieving, so, he is not an option, I am a little lazy not to drive 50km to the next one who has been great in the past beyond my expectations and make initial inquiries here on Thai Visa, to his credit (the next closest dealer is a good guy) , I am just seeing if anyone has had one installed or even if they are available. As long as the "Genuine" wiper parts are not a rip off, and my experience with original Carryboy parts are they are reasonably priced so far, I would like to hear how much it cost and if it worked as you expected thanks?

Oh, and how much was the wiper to install please?