alpine cd player

Usually all CD changers have to be mounted close to horizontal or close to vertical.  There's a switch or dial on the side of the changer that you have to adjust according to whichever mounting method you use.  The variance from true horizontal or vertical that each changer can accomodate is usually different among manufacturers.

In general I'd recommend against installing a changer on the back of a seat.

I think the problem you're having is that it's not properly grounded.  When you moved the changer, and had to disconnect (or unscrew from the car's chassis) the small black cable, did you reconnect that once you had the changer mounted?  Not having a proper ground connection can and will cause the changer to not turn on.

If you do decide to mount it on the floor, double (no, triple) check the area underneath where you're mounting it--you don't want to screw/drill in to a gas tank or fuel line.  For that reason, and because it is more likely to get damaged from stuff being placed on top of it, I try to avoid floor mounting in most coupes and sedans.

I usually try to mount it horizontally on the underside of the rear deck, or vertically on a rear side panel (usually only works on suvs).  But if I understood your previous post correctly, you don't have room on the underside of the deck, right?  Try to squeeze it in up there if possible.

Of course mounting orientation and placement all depends on the model/type of vehicle and what you have in it.  What kind of car is it?