i've had both, the backlights for the instrument cluster on the 99's should be bluish, while on 01's it's greenish...i couldn't tell you why it may be the wrong color....
The switch came in 01 for all instrument clusters. My buddy from high school had a 2000 GT with bluish cluster lights. My old 2002 GT had green cluster lights.
all of you who say you have green in 99's...you need to find an 01+ (any mustang) and look at the backlights on it...i'm betting your lights seem greenish to you but if you compare them to a newer one, they'll look blue...