The alpine cage are designed diferently to most. you can if you want do the whole remove cage from unit and insert before fitting the actual radio into it, bt if you look at the many finer tabs on the alpine cage, you can actually just connect your wiring loom(s) (be sure to switch the red and yellow around when connecting them on the alpine looms as you may find theyr not connected at all to begin with)
also sumtimes there is no use for the in line blue wire on the alpine wiring loom which could often cause other conflicts with the vehicles electronics so therefore i would normally chop it n tape it up out the way.
once all that is done, just slot the alpine in as it is even when the cage is stil on the unit as it will clik in place by itself if done properly.
All the best with it.