I ended up getting DIC from a 2003 GTP and it worked fine, even though Morad fought hard to convince me the wiring was different from '98-02 to '03.
It turned out that the chip under the LCD in the '99 was shot..... Luckily it wasn't one of the Delco chips, but an LCD driver made by Philips. It was a pain because I had to desolder the LCD to even get to it. If anybody cares, it was the Philips 16206747 which has been replaced by the NXP PCF2111CT. The Digikey part number is 568-1065-1-ND