How to get Collision coords

Ok.. I didn't give enough information with my first post... let me try again.

I have a sceneobject(A) that is colliding with another sceneobject(B). I would like to know where A collided with B... on B's surface. Then I want to mount another sceneobject(C) where the collision took place (coordinates) on B.

I'm assuming I'll need to use customcollision, but not sure where to start.

-Tim