I have Neons instead, but yeah I have underglow too. The back one burned out because of there was pressure between the rear bumper and the backup tire holder cavity, so I replaced them with LED Strips and they work just as good. Anyways yeah I'm in the same situation as you, I have a Corolla S too, here's some pics of mine, installation was fairly hard since I couldn't find any decent holes to hold the Neon Tubes, but LEDs should be easier,
http://i62.photobucket.com/albums/h1...g?t=1287197715
http://i62.photobucket.com/albums/h1...g?t=1287197677
http://i62.photobucket.com/albums/h1...0918102212.jpg
http://i62.photobucket.com/albums/h1...0911100408.jpg
The last 2 are bad quality, but you'll see the point. They look pretty darn good,
Corolla's can get girls too

mine did
tell me how the HID's work out for you, I have Sylvania Silver Star Ultra's giving me a good 4300k WHITE lighting, and it's serving me pretty well, but a little more blue would had been good.
and an idea, since I already have the underglow on, I'm gonna piggyback wires from those to the inside and put in the extra LED Strips I have and put them in as footwell lighting, it'll also be my indicator lights if my neons are on. just a suggestion
and I know the car looks black, but it's actual the dark blue one.