I have been using my '99 Ford Windstar to transport my visiting therapy mini, Sunshine, for over a year now. I got the whole setup idea from a friend who has been doing it for 8 years.
Set up: remove the middle seat, lay down a heavy-duty plastic tarp (paint tarps or shower curtains both work; the latter is pricier). I use duct tape to secure the tarp (about 4x6') up about 6-12" on all sides, including to the back of the driver's seat and the rear bench seat. I bought some 1" thick puzzle-piece exercise mats from a friend's karate school, taped two together permanently and cut corners to fit into the space exactly. So the mat goes down atop the plastic tarp. A few towels on hand can soak up any liquid that comes out. All tack and equipment is organized behind the rear bench seat, including a hanging harness bag. Finally, the cart goes on a bike rack on the rear of the minivan (shafts straight up, seat back just below the bumper, tires toward rear).
Ramp: At first, we used a ramp (see my pix at mustardseedminifarm.com) but after a couple brief training sessions, we taught Sunshine to jump in, and he has been doing it ever since and does a great job being careful. A lot easier to not have to mess with a ramp!
Pros: no trailer, you can do yourself (I am 5'1" and can do everything without help), inexpensive equipment, compact operation/easy to park and take anywhere, and fun to drive through McDonald's with a horse over your shoulder!
Cons: Poo happens! And the other stuff! I have least stress when I bring along a horse-knowledgeable helper who isn't afraid of the smells and knows the tell-tale signs that something's about to come out of the horse and can CATCH it in a bucket! We line our bucket with a bunch of plastic grocery bags, and tie them up when used to keep the smell contained while we drive. My friend who has more experience has been more successful potty training her mini, but I think that is easier said than done. Mine prefers to poo right away, and often throughout the trip. It is a lot of clean up unless I have a good helper along.
Truth: I am considering switching to a tiny trailer that could be pulled by a van or suv, since my carpool kids think my van is a bit too barnyard for their tastes and the cleanup would be easier.
Good luck whatever you decide to do!
Lisa Schutter www.mustardseedminifarm.com