I FOUND OUT EVERYONE!!!!! IM THE BEST, jk but i found this and i think it'll be useful, espicially for you nick,
" I have a 2003 Mustang GT and I saw that you guys said the bulb that the man above was posting for his 98 cobra wasnt working in the 99-04 mustangs. The reason why is because our (99-04) clusters are electrically controlled theirs (94-98) are mechanically controlled.
Anyways the bulb size for a 99-04 mustang is a 74-GHP3 you can get it from the above website that he posted. Your bulb size may not have a "G" in it mine has it because I ordered bulbs that were green so for example if you order blue bulbs for your car it'll say 74-BHP3. "
HOPE THIS HELPS!!