The harness was another part of the project that I thought about for a long time. Since this wheelchair is for back-end support, the front-end harness isn't really taking any weight - its main purpose is to keep the cart frame positioned correctly. Another important function - it stops the front of the frame from lifting and flipping over backwards if your dog decides to slip into reverse gear.
Some of the dog wheelchairs out there include custom harnesses, and some list harnesses you can buy separately. These are certainly an option, but they are expensive. I decided a conventional on-leash chest harness would do the trick. These harnesses have two adjustable flat nylon looped straps, for the neck and chest, joined top and bottom by thick nylon straps - one between the shoulder blades, the other along the chest. They typically have a metal ring on the top for attaching a leash.
Click here to see the harness I used, but there are others that are much the same. I found this one for half price at PetSense, a discount pet supplies store. Be sure you get the right size; it should adjust to be firmly around your dog's chest, but not constricting in any way. If you can easily insert a couple of fingers between a strap and your dog's chest, that's probably fine. Make sure your dog's neck strap is not uncomfortably tight. Remember - this harness isn't taking much weight. Depending on your dogs size, body length, and neck length, you might need to move the leash ring or insert another onto the harness at a better spot. You'll be able to figure this out once your dog tries the wheelchair for size.
Also - make sure the harness is fitted correctly! With some models, it's easy to mistakenly fit it upside down, which will be very uncomfortable for your dog.