In IM 12.6 the JCS was changed and is now using the OSGI bundle mechanism to load jdbc drivers such as Oracle, Sybase and Siebel.
The documentation explains how to build an OSGI bundle and does also explain that the relevant JDBC driver would need to be put manually into C:\Program Files (x86)\CA\Identity Manager\Connector Xpress\lib
In JCS you have the possibillity to write JS script in conjunction with Operation Bindings. Here you can basically write your own connector.
But if you are trying to access the jdbc driver with for example
java.lang.Class.forName( "com.sybase.jdbc3.jdbc.SybDriver" )
you will get
getJDBCConnection ERROR: java.lang.ClassNotFoundException: com.sybase.jdbc3.jdbc.SybDriver
in the endpoint log located in a subdirectory under C:\Program Files (x86)\CA\Identity Manager\Connector Server\jcs\logs
As far as I know, this is due to the jscript engine does not work well with OSGI.
To fix this, you would need to add the JDBC driver in question into C:\Program Files (x86)\CA\Identity Manager\Connector Server\lib
So for the error above, you would need to copy jconn3.jar to C:\Program Files (x86)\CA\Identity Manager\Connector Server\lib and restart the Java Connector service (Net stop im_jcs ; net start im_jcs)