My GF has an '03 Protege 5. I was wondering if this is considered the predecessor to the Mazda3 5-door or not. IMO, the Protege 5 and the Mazda3 kind of look similar and seem to share similar exterior colors.

 

The Mazda 3 doesn't have much in common with the Protege. The Protege came out of the Mazda design house and based of the old Mazda B/323 Platform. Your Protege 5 may share some of the Ford electronic parts, depending on it's build date. When Ford took over, they started building the second generation and last of the American Escorts off this platform. A good portion of the factory tooling ended going to KIA to make the early Sephia's. The Mazda3 shares its platform,the FordC1 Global platformwith the Volvo S40, V50, C70, and C30. The European Focus C-MAX, and the MAZDA5.Suspension is the standard Ford E-link at the rear, with Japanese manufactured struts and coil springs at the front with an antiroll bar. Engines for the 3 are the Mazda MZ series, therefore pretty reliable. Transmissions for both your P5 and the 3 are unfortunately Ford products, and will be the weak link. The Protege would have either been assembled in Mazda's Hiroshima or Hofu Japan Plant. The 3 is exclusively assembled at the Hofu facility.