Randy Lervold’s old RV-3 site has good pictures - the servo is put on the aft deck, under the vertical fin, with a short Bowden cable going out through the stab , through the elevator, and connecting to the tab.
 
Here is a picture of my RV3 electric trim
 

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Awesome! I anticipate upgrading my manual elevator trim in my 9A when I upgrade the panel. Seems this will be an easier mod than putting the servo in the elevator.