I want to do RAD with C, but the only ones I can find only use C++.
You may have to define what you expect when you use the term RAD. Technically is is a methodology agnostic of language (see http://en.wikipedia.org/wiki/Rapid_application_development, to understand what I mean).
If you mean "visual development" (which is the context in which it is applied to tools such as Delphi and C++Builder, primarily as marketing buzzwords and because Microsoft had already essentially trademarked "Visual" with respect to development tools), then use of C would require an application framework and tool to support this. There is a reason why such tools are usually implemented with object-oriented languages; it is a much more natural fit for GUI elements. This being the case, a tool vendor would be unlikely to choose C as the basis for such a tool.
Also it may be useful to understand what platform(s) you wish to target.
A judicious bit of Googling threw up a couple of things that may fit your requirements: