Full coverage and protection of effect areas on the point of origin, not on the launcher
The breath of the dragon goes through two 'steps'; the spell is first cast on a creature targeted by a touchscreen. Standard touch targeting rules are applied. Then, the affected creature (as long as the spell lasts) can use its action to produce an area of effect that deals damage.
The relevant extract from the Player's manual with regard to the effect areas:
The effect of a spell grows in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location in the effect area, that location is not included in the area of the spell. To obstruct one of these imaginary lines, an obstruction must provide total coverage […]
— Player's manualp. 204
Who like The breath of the dragon There is no exception to this (some other spells say "this effect extends around the corners" or whatever), the area of the spell's effect is blocked by corners.
However, the area of effect of The breath of the dragon is a cone from the affected creature (the one who needs to use his action). The position of the wheel does not enter the picture.
Similarly, a spell with an area of a sphere can affect a creature that has full coverage of the caster, provided that the origin point (in this case the center of the sphere) has a clear line with this creature.