How to set a wiper on a center rest while keeping the equal left and right stroke? ?

Yes you will need to relocate the wiper transmission, then re-size the link arm that runs it. It may take a little bit of fabrication to get it done, but i do think you will be able to. Examine the stroke of the wiper motor and pick a position for the wiper trans, then cut the transfer link to the length you need. Make sure the wiper motor comes to the rest stop so it wont keep re-cycling. It wont be able to find the stop position if you do not somehow change the transfer to linear back and forth. The motor is set up for rotary drive and will keep cycling it if does not sense the wiper arm as stopped against the rest.