The 9-5 ignition key contains a battery that allows the key to start the car and operate the remote locks.  Getting the case open is easy if you make a simple tool, but can be really, really difficult if you don't. I replaced the battery in my spare key and afterwards I couldn't get it to work; I'll try to get more reliable info on how to reactivate the key, but in the meantime I'd recommend caution before replacing the battery if you have only one key ... you might end up walking to the dealership!

 

There's a small hole in the side of the key.  The idea is to insert a small rod into the hole and push to release a latch inside.  The write-ups that I've seen say to use a paper clip to deflect the latch while prying with a small screwdriver, but that's pretty ridiculous - this latch is pretty formidable, and it takes quite a bit of pressure to deflect it enough to release it.  Aside from the pressure required, there's also the matter of holding everything.  The use of an awl is also mentioned, but the sharp point will just stick into the latch (it's plastic) and tend to capture it rather than releasing it.  See the pictures below, particularly the last one, to see how beefy the latch is.  Text continues below the pictures.

 

 

So the trick is to find a very small nail - nails that small are called brads, usually - and drive it through a piece of wood.  I used a 1" long brad that slipped into the hole nicely.  I used a hammer to drive the brad through a piece of 3/8" plywood.  A strip a couple of inches wide and a foot long or so should work fine.  Drive the brad in about half an inch from the edge of the wood strip.

Now file the point off the brad so the end is flat across the entire diameter, then round the edges off just a tad to remove any sharp edges.  

Lay the board down on a counter, sturdy table or work bench with the nail sticking up - position it so the nail is pretty close top the edge of the work surface, so you can get access to the key.  Have a small flat-blade screwdriver handy.  Now, just slide the hole in the key down onto the brad, push down firmly ...

... and pry the case open with the small screwdriver.  Pry the seam just above the hole.

It takes a good bit of pressure on the nail to release the latch, but it's simple to apply - just put your hand on the key, get the weight of your upper body over your hand and lean into the key.  The key will pop downward suddenly as the latch releases when you pry the cover - it won't take much prying once you press hard enough on the key to release the latch.  

Here are the components:

The battery is retained by two thin metal clips and a plastic tab.  

There's a little tit on the plastic tab - pull it toward the blade of the key with your thumbnail and the battery will pop up.  

A bit of pressure on the battery between the two metal tabs should make the battery leap out of the socket - if not, it will happen when you release the pressure.

The battery is a Panasonic CR1632 (or equivalent).  

You'll probably have to order it on line.  I got it from radioshack.com.

To install the new battery, the larger side (positive, the side with the lettering) faces outward.  Press it under the two metal clips - you'll have to press down a bit too, to depress the contacts underneath the battery.  Then just press the front end down to snap it into the plastic latch.  Put the two tabs of the plastic back piece into their slots and press the latch in until it snaps into place.

Supposedly you just press the Unlock button 4 or more times in the vicinity of the car to activate the key again, but it didn't work with my backup key.  I have yet to determine how to activate it (I'll probably end up at the dealer).