Even if it is on you will not see anything. The OBC "bible" is here
JBs OBC Primer if you scroll down to the table you'll see that it is only on a 1997-2000 where you get the temperature display without doing any additional work.
On an 04 (as with my 03) you need to pop some wires in to th econnector on the back of the cluster. It isn't difficult, just make sure you get the tight wires. Order two of the "010 VW/Audi Wires" from Suncoast, they're about $3 each, cut each one in half and you have all you need for full OBC control.
If you only want the temperature you don't really need a switch even, you can just cycle it with the bare wires then tie them off. If you want control you can either add a dash switch (or two if you want to use the speed gong too) or use regular momentary switches from radio shack.
Chris.