Arrow, she says she's having trouble registering for some reason, but here's the mini version . . .
Lots and lots and lots of ground driving. Get the horse used to the person being behind, not on the horse. Then introduce sounds behind her while ground driving. Then introduce some weight-a tire hooked to a safety hook so it can come off quickly if needed is the first type of weight. Then have someone pull on the tire to add some resistance. All of this should be done in a regular bridle. Also all of this should take months-not days or hours. The horse has to be totally dead with all this before adding poles to the sides and incorporate with all of the above. There is nothing more dangerous that a runaway horse and buggy, so patience and going slow and careful are most important.
If you would like to chat with her via e-mail and get more information, send me a PM and I'll give you her e-mail address.