They are inside the metal mounting case thing tight as a crab's backside.
If you have unlocked the latches with the keys and cannot get it to come out without exerting too much force, no matter how much you manage to keep it square,.....
You'll need to take it out with the mount on it.
You will be able to do this by inserting a very thin blade around the outside of the metal case.
As you do this the locking tabs, which are bent out from the mounting case, to stop it coming out, will need to be pushed flat back towards the case. These tabs are all parallel to the front face so this isn't as hard as it looks.
Once you have freed a few of them off it will start to move, so you will see which places you still need to work on.
I've had to do this with two LR head units so I think the way they fit them is a real sod!
Once you get the whole thing out you will probably be able to separate the unit from the mount.
Best of luck.