When I hit the connect to the Constant Contact API I am getting this 400 - Invalid redirect_uri. I have no idea how to correct this error. Any idea?
The redirect_uri needs to match the setting in your API application as well as in your integration. I would encourage you to check our API documentation to ensure your oAuth flow is set correctly.
You did not mention which version of the API you are using so here are the current URLs to both versions.
View API documentation, code samples, get your API key.