First, the good news... as far as seating and controls, at least for lower tech levels, wings vs. wingless probably won't make much difference.
There have been some other questions regarding seat design for species with tails or other unusual body plans before, but those were generally for bipeds.
Your real issue is that quadrupeds aren't as adept at manipulating things as bipeds. In fact, depending on how they move, they may have very limited sideways mobility of the forelimbs compared to humans, which suggests that human-style steering wheels are not going to be usable. They may still develop rack-and-pinion steering, but they will likely need more gear linkages. The best I can envision offhand is a sort of bicycle-like crank operated with one forepaw. The other foreleg is going to be needed to operate the throttle and brakes, but fortunately these don't need to change as much from human design. Note that this means that power steering (and later, steer-by-wire) is going to be really important. In fact, steer-by-wire may cause a complete redesign of vehicle controls.
But let's skip ahead to seating for a moment. The simple solution is that they sit on their haunches like many quadrupeds do, and lean forward on something so that their forelimbs can be used for manipulation rather than keeping themselves upright. This will become especially important when crash mitigation starts to matter, although there's a little unexpected bonus here; being supported in the direction of forces is preferable. If not for the little niggling issue of wanting to see where we're going, we humans would actually be safer in rear-facing seats.
However, this only gives them use of two limbs, and in the early days of slow vehicles (and before suspension is developed!), I suspect they might actually prefer to hang from suspended harnesses. With some sort of elastic material, the "seat" can be cushioned against a rough ride even if the vehicle as a whole isn't. This would allow them to also use their hind legs to control the vehicle.
Fast forwarding to "modern" day, I can imagine something like this: A dragon walks up to his vehicle and the automatic door opens when he taps a sensor on the running board (or waves his paw under such a sensor). He climbs in head first and rears up in order to settle himself on the mostly upright couch, which supports him at the waist, sternum, and under his haunches. (More support is better, but his chest needs some room to expand when he breathes.) Automated restraints wrap over his back (because he can't reach!) to secure him in place for safety. His hind paws locate control pedals while he places his fore paws on the steering yoke. He moves one foreleg sideways a bit to press the starter button, then puts the paw on the adjacent shifter and shifts to 'drive'. A press with a hind paw on the throttle starts the vehicle moving, and a gentle pressure on the steering yoke causes it to turn.
You could swap the steering and throttle/brakes (more like an airplane); the key points are:
- His body is supported in a way that lets him use all four legs to control the vehicle.
- The seat needs to blend support against gravity with support in case of an accident. A horizontal seat is more comfortable, but all the forces in an impact will be transmitted to the shoulders. A vertical seat is best in an accident, but needs to support the occupant's thighs (much like a human chair does). At least for vehicles, think of their seats working like a human sitting on a chair backwards.
- The steering is almost surely steer-by-wire, requiring only minimal movement. (For a yoke, I'd guess no more than ±45°, if that.) You could use either set of limbs for this, or even the tail. (Using the tail would be much harder before steer-by-wire is developed, but afterward, would be a really cool and probably more natural way to control a vehicle. This might leave their forepaws entirely free to work other controls.)
- Auxiliary controls need to be in easy reach. Remember, depending how you design them, quadrupeds may not be able to reach behind themselves or to the sides very easily, if at all.
Once you do have all-electronic controls, you have the possibility of using the wings as an additional means of control. The issues here are that wings may be delicate (best in an accident if they are tucked in rather than extended) and figuring out how to build an interface for them may be tricky. (Plus, only half your dragons could use such controls.)
One caveat: I think their bicycles would have to be something of a cross between bicycle and skateboard, i.e. the front steering mechanism also needs to be load-bearing. Alternatively, the might have vehicles that are more like open-framed cars, in which the rider is suspended.