Hi - I recently replaced the factory head (yuck) with a JVC KD-G611 unit and all is well (fantastic actually - it even looks factory). The output power does not really matter as you would be mad not to plug it into the factory amp. You need to get a wiring adapter (void warrantee otherwise) and an antenna adapter. These are very cheap and any good audio shop will advise and install - the factory amp really kicks with the JVC (cool).

Enjoy