Similar to what pythonjosh has suggested you can also just cut a USB cable long enough to reach from the front USB port to where your fan is located in the back.
You can then carefully strip the insulation from the red and black wires in the USB cable. Turn on the PS2 with the USB cable plugged in and touch the red wire from USB to the pin of the PS2 fan's red wire, and black USB to fans black wire pin. If the fan starts to spin that is great. Proceed to the next step.
If it doesn't then check your wires aren't crossed black to black, red to red. Or you can see if you have other small case fans that will start with this method. This way only provides 5 volts to the fan so it is not a permanent fix as most fans won't blow much air at 5v. You really should have somone change the fuse.
Now turn off the PS2 and disconnect the USB cable.
Next strip the insulation from the wires on the fan and twist the red wires copper ends together tightly. Do the same for the black wires. Then electrical tape around the red wires and make sure to cover all the copper so the red and black wires don't touch. Then wrap tape around the black wires. Make sure no copper is exposed.
If done carefully and neatly you can route the wires inside the PS2 and back to the fan. Or you can leave the wires hanging out the fan vent hole and route them to the USB port externally.
Your PS2 may still overheat after 3 or more hours of play in a enclosed entertainment center, so leave it out in the open for best results. Since this is only a temporary solution, I don't recommend leaving it this way for very long.
For pictures of a similar fan modification I have done to cool my PS2's HDD see the link in my signature.