What exists today
API access is listed as part of the Enterprise plan on pricing. In practice that means an arrangement made with us rather than a key generated from a settings screen, and what it covers is scoped to the case being solved.
The interface the application itself uses is authenticated per dealer and is not documented for third parties. It is not a stable contract, it changes when the product changes, and anything built against it would break without notice. That is the honest reason there is no reference here rather than an incomplete one.
What a published API would have to settle
Authentication and how a dealer grants access to someone else's software. Which resources are exposed and which stay internal. What is guaranteed not to change, and for how long. How a change is announced. Those four are the difference between an API and an endpoint, and none of them is answered yet.
If you are building something
Tell us what you are trying to connect. A concrete integration with a named customer is what moves this from a plan to a commitment, and it is also the only way to find out whether Enterprise access already covers what you need.
For the channels Worldwatch connects to on a dealer's behalf, which is what most integration questions turn out to be about, integrations is the list.