Not finished, no, Jan. I'm travelling a lot right now for work right now so modelling time happens in little bits and pieces. This is actually not too bad for harness because that comes in little bits and pieces as well. Eventually it will have plough, shaft (single) pole/wheeler (pair) and and leader (the front pair in a team of four) variants.
Long explanation for those actually interested who don't know already:
Plough harness only HAS to consist of the hames, which connect the weight of the plough to the part of the shoulder which allows the horse to push with its whole strength.
The plough chains connect to the hames hooks, which are the upside-down-pear-shaped hooks. The collar spreads the force from the hames across the horse's shoulder, to prevent it from getting sore (when properly fitted anyway). Hames straps or chains hold the top and bottom of the hames together, keeping them in place on the collar.
Aside from your means of steering the horse, which can be purely voice, you do not NEED anything else at all, and I have seen ploughing achieved with nothing but the hames and collar (and a bridle, but the plough lines- the ropes connected to the bit- were slack). People usually add a back-band, which lies across the horse's back in the same area as a saddle and holds the plough chains off the ground when there is no tension to keep them up anyway. Some back straps have nothing running under the horse's belly to keep it in place, except perhaps a strap or straps fixing it to the collar.
When pulling a wheeled vehicle, you add britching or breeching (terms vary by country and area) for the horse or horses connected directly in front of the vehicle. At its most basic this is a broad strap around back of the horse's bottom, held in place by straps across the top of the horse's bottom (loin straps or hip straps). The horse sits back into this strap to brake the vehicle or move it backwards (some vehicles do not have brakes but rely on the horse, especially agricultural vehicles where the driver may not even be on the vehicle at all but might walk instead). The straps are prevented from sliding off backwards by a strap along the line of the horse's back that fixes to the forward part of the harness (back strap).
There are two main methods of attaching the horse to a vehicle- either via shafts that attach at the horse's sides, or with traces that run back to a pole or other fixing that sits crosswise behind the horse. Vehicle harness includes a girth and 'saddle' of some kind, although you cannot usually sit on this kind of 'saddle'. A martingale is also usually added. This is not the kind used in riding, and does not connect to the bridle at all. Instead it goes between the horse's front legs to connect the collar to the girth. A chain may be fixed between shafts which then rests over the horse's back, with a thickly-padded cart saddle fitted to the harness which gives a channel for the chain to sit in. Alternatively, a loop (tug) is added to each side of the harness and the shafts run through those. Additional fittings fix the hames and britching to the shafts. A band is added underneath the horse's belly to stop the shafts from rising upwards.
Tugs and/or cart saddles are not needed without shafts. Metal loops called terrets are usually fitted to the top of the saddle (which is really just a padded band) for the reins to run through back to the driver. The hames hooks connect to long traces which lead right back behind the horse. The harness usually has loops or chains connected to the saddle and/or the britching to hold the traces off the ground when they are not under tension. The traces connect to the vehicle, either directly (for some designs, when driving a pair) or indirectly via an arrangement called a whipletree, which... I need to model to explain, I think! You remember the sort of mobile made of a bunch of horizontal sticks, each hanging by the center with a string hanging off each end for the next stick down to be tied to? Like that, only lying flat, and the traces behind each horse attach to either end of their own stick, with the center part attaching to the ends of the bigger stick, which then attaches to the vehicle. Whippletrees help balance the load between horses, and you can connect them off-center to give a horse less work to do- if you're training a younger one to a team, for instance. A pole runs up between the pair of horses from the vehicle, and the front of the collars connect to the ends of the pole. This allows the horses to push the vehicle backwards by transferring the weight of the vehicle down the pole to the collar and on in turn to the britching.
Pairs in front of the wheel pair do not need britching, because they have no means of transferring their force backwards to the vehicle- they have no center pole (no load-transferring pole, anyway). Show harness may include britching anyway for decorative purposes, but functionally it has no use. Vehicles large enough to NEED big teams also tend to be fitted with brakes! The traces of these forward teams connect to a whippletree that connects to the front of the pole between the wheeler pair. Because it is attached centrally this way, you do not have to have the same number of horses in front as behind- you can have three instead of two (a pickaxe) or a single (a unicorn).
Some of the other harness parts used to be functional but are now primarily decorative, a couple of parts are to do with how you communicate with the horse rather than how you pull the vehicle, and some are purely decorative- such as harness brasses.
Okay, if you were not interested you can wake up again now!