In October 2009, Michael Nash contacted me about publishing his work on disassembling the TI-89 Titanium, comparing the two major hardware revisions, and use electroluminescent wire (EL wire) to add a backlight. At long last, I have gotten around to translating his PDFs and images into a news article and forum topic. Enjoy Michael's work, and click through for the entire tutorial from Michael, including tons of instructions and screenshots.
Enjoy this long-suffering piece of research missing from community documentation (click through for disclaimer). Perhaps try it with a TI-83+, with its huge empty spaces in the case, or a TI-84+, which has a very similar internal layout to the TI-89 Titanium!