Everything I've been able to find indicates that the hardtop from my 96 ought to fit an 03. The local Mazda dealership parts guy says it's not compatible.
The same dealership also claims it takes $300 in parts and $90 in labor to equip a 2003 to accept a hardtop. The $90 in labor sounds reasonable, but $300 for parts?
A side-latch kit is about $60, and the frankenstein bolts can't be much. The only additional parts I can think of would be defroster wiring, but the 2003's already have wiring for rear window defrosters don't they?