I ssh -X username@server to a Solaris 9 box as a normal user and su - to root and run /path/to/program and it says "Error: Can't open display:"

Does anyone know what I need to do to be able to run this program? Thanks.

If you su to root I think that the variable DISPLAY will not be set, while it will be set in the environment of the user which connects via ssh (you can try an to check it). If that's the case, you should set it to whatever it should be, such as:
for example
Hope this helps,
Enrico

Thanks for your help. As normal user when I do echo $DISPLAY
I get localhost:14.0. I su - to root and do export DISPLAY=localhost:14.0
and when I run the GUI app it says -

X11 connection rejected because of wrong authentication.
X connection to localhost:14.0 broken (explicit kill or server shutdown).

My local machine's echo DISPLAY is set to ":0.0" and tried to set the DISPLAY on the server as export DISPLAY=/my-local-machine's IP:0.0/ and run my GUI app again and it hangs in there - I don't see anything.

Running the GUI app as a normal user still works fine.

Any further help would be much appreciated. Thanks.

The behaviour you observe is expected. The X server thinks root is trying to hijack your user's X session.
Try using "su" not "su -" to keep your environment.

Thanks all for your help. I can now get the GUI.