Rear Wiper Intermittent only

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I have a 91 Plymouth Grand Voyager Van with two buttons for the back wiper. One to wash and one for intermittent. They both work but the intermittent on will not go continous. Note: these are push buttons and no extra adjustment is available for the intermittent. So I phoned a dealership and asked them if this was normal or could you change the intermittent to continuous. They said it was normal and there is nothing that you could do to change the way it works.

What i need to know is if someone else had this problem and fixed the rear wiper so that you could do a continous wipe or both (intermittent).

Thanks, David.