Depends on the Year & Model.

On my 2000 TL, there actually are a pair of filters - Fram sells the
replacements in pairs.  Dealer wanted about $100 to do it.  I did it
for about $15.00.  

They're behind the glove box and fairly easy to change if you have the
factory shop manual as a guide, or can figure out what to do.  The
first time you need to cut out a plastic piece with a wire cutter
(etc.).  It gets reused.

Here is a link to a very nice DIY.  Following this it took about 15 minutes
for my 02 TLS.  This link should work for you.
http://tl.acurazine.com/forums/showthread.php?t=107821&highlight=cabin+filter

Here is a link for the 04 and higher TL's.......
http://tl.acurazine.com/forums/showthread.php?t=123884&highlight=cabin+filter

- Mike