I have a cls550 with two key fob with metal keys. The batteries need to be replaced on both fobs One of the fobs is broken and needs to be replaced.

I looked at some YouTube videos about changing the battery(ies) but I can't seem to get the fob apart to get to the battery(ies). Any suggestions on getting the battery(ies) out and on where to get a new fob? Should I go to the dealer? I heard a key can cost as much as $500. I really don't want to spend that kind of money for a key. Should I buy a fob off of ebay and stick in the metal key from the broken fob.

Does anyone know what happens if the battery(ies) in my fob die(s)? Will the key still open the door and start the car?

Thanks all for your help.