How to configure OTDS to use the Content Server "Admin Server" for Authentication.

I am replicating my orgs production setup for our test environment and am so close except for the authentication configuration.

3 servers in each environment. 1 = CS, 2 = CS Admin (search indexing, EFS, auth), 3 = OTDS.

In production the OTDS resource syncs to the Admin Server for auth, but I just cant seem to figure out how to configure the test environment to handle OTDS auth through this 3rd server. I keep getting a SOAP error when I plug in the Admin Server address. Even more interesting in Production, the integration setting on CS points to the OTDS server. I must be missing something.

Any help is greatly appreciated.