I took the cart out again today and both Rumer and Percy were much more inclined to come play than make a big deal out of it. I had to be pretty far away in order to get the whole area on the video so the detail isn't great but here is the basis of what I did:
I began as I did before, just pulling the cart into the paddock and both ponies immediately began following the cart. Rumer remembered that the place to be was next to me so even though I did not slow down, she quickly caught up with me to be alongside the shafts. I clicked and treated her for remembering and being right there. After a couple times, it soon became a little difficult to keep going because I had two ponies right in the way and not at all worried about the cart.
The video is too far to hear the clicks but you can see when I treat and at one point I bump her in several places with the shafts as I did last time and C/T her for standing still. Only once does she startle and pull away but I just ignore that. I also take a little jog around and both are happy to jog along with me, getting used to the noise and bouncing of the cart. Maybe Percy could be driven!! I also put the shafts up a couple times, so they can become accustomed to that.
As usual, my biggest challenge is getting Rumer to stand still so I can do things to her rather than her being the one to be clambering over me. So I brought the plywood mat in. I had no real goal here- just playing and seeing what I could accomplish. I knew it would be a challenge for her to focus on the mat work with the cart right there and Percy wanting to play too. But in fact she was fine with it all and got right on it. A couple time I repositioned her by just having her target my hand.
Percy became much more of a challenge to my focus! I got Rumer lined up on the mat a couple times, with the cart right in place behind her. I was able to leave her and walk back a little ways toward the cart and have her stand.
You can see the video at http://www.youtube.com/user/BookendsPonies