Lesley,
Since the Aries has bevel gears to connect the vane to the servo, and this is where the negative feedback is put in, I think you'll have to connect to the vertical push/pull component. Clamp a 2 or 3mm Dyneema line to the centre of this, and lead it through a block hanging from the vane turret, and through a block near the bevel gears. Then it's a question of leading the lines forward to a place where they can be similarly attached to the push/pull rod of the tiller pilot.
Any gear like the Hasler that has a little tiller at the top of the servo is a bit easier - there is a natural connection point there, at the end of the tiller, with the lines lead out through blocks on the frame and then forward.
I did all this years ago when I had a trim tab, and wasn't very happy with the result, but that was probably the trim tab's fault. I really must get around to trying it on the servos.