I have a 2001 RX300 Silversport and I noticed that the rear vents don't work at all. Is there a button up front that I have to press?

 

There are no rear vents....

 

Try putting it on the upper vent setting and put the fan on high (of course make sure the vent are open in the back too). If you close off one of the front vents it will blow harder back there too.

 

There's vents, on the center console between the two drivers seat.

 

Oh my gosh! I feel so stupid..... yes, you need to have the air control on head to get the most air out of the rear vents. If you have it mixed (head&feet) then only little bit of air come out. Grandma complains about that (hot flashing).