I had come across this a while ago, prior to buying a new fob without inwards, so hoping to swop over old key contents to new one, whilst new one sort pops open, the old one is almost unbreakable into.....help!!!
The original cases are plastic welded shut...only way to get into them is with VERY VERY careful cutting....best just to replaced the worn rubber pads, battery and battery cover with seal..
The likelihood of busting the circuit board is high....and even if you do get in, you may find the tiny microswitches for the fob buttons have come off the board and rerendered useless...