my 2000 cadilac catera overheats unless i run the heat whats wrong

I am providing you with a diagnostic walk through that will cover the most probable, and genrally the only, casues for overheating.
you should also have the trouble codes pulled. Any Advance Auto ro Auto Zone will do this for free.

Here is the link;

https://www.2carpros.com/articles/engine-overheating-or-running-hot