recently I got an alpine cda-9855, its the same size as my previous deck and I switched them, my probvlem though is that my alpine wont lock, it wont lock into it's security cage, i can push the deck all the way in tyo where it shud lock but it doesnt, is it possible i have the cage lock for the deck in backwards or upside down?
You need to use the mounting cage that came with the CDA-9855, that appears to be what your problem is.
If it's upside down, it really shouldn't matter, though you can always reverse it if you need to. Also, if it's in backwards, it is extremely obvious because the lip is at the back. That could also cause the unit to not lock into place.