key fob programming for 350z

https://www.youtube.com/watch?v=ufYg3drY1p0 also ............... http://my350z.com/forum/body-interior-exterior-and-lighting-diy/170334-how2-remote-key-fob-programming-2.html

my key fob is a case from Autozone. I took the insides out of the original fob cause the original fob broke and wouldn't stay together. I went to autozone and bought a new case and put the insides in and it worked. Then the replacement from autozone also broke so I went and bought a new one and now my fob is not working at all. I tried all the instructions on the videos but my fob will not program the way the instructions are telling me its to work. I do the whole bit but what doesn't happen is when the flashers go on and I turn the car on without starting it and hold the button waiting for the flashers to flash one more time to set I never get the flashers to go on again. so when I push and hold the button they just stop flashing and wont program. What I don't get is why when I switch cases only it doesn't keep working. I didn't undo the battery or anything just took the complete insides and put in new case and they don't work.