All of the sources that I could find, state the same thing: The Plus upgrade gave the Harrier II a new radar, which allows it to launch advanced missiles like the AIM-120 and the AGM-84. Nowhere does it actually say that a Harpoon was ever fitted on a Harrier though.
So, and this is merely speculation on my part, I think that the new Hornet's radar gives the Harrier the ABILITY, in theory, to fire the Harpoon. In reality though, they don't need to try to fit Harpoons on Harriers, because there are more suitable platforms for that purpose.
In Spain's case, they operate Harriers off of their carrier, so they wanted them to have some anti-surface capability. However, seeing as they would probably need to further upgrade the aircraft to allow it to safely carry Harpoons, they dropped the idea. Again, this is purely speculation.
If this is the case however, it means that the Spanish Harriers, really have only Mavericks to use against surface ships, because I couldn't find a shred of evidence that the AGM-119 Penguin was ever fitted on a Harrier either...
< Message edited by dontpKaniC -- 12/12/2015 8:40:09 PM >