The backbox key is found dangling on a hook on the inside of the coin door or on a hook fastened to the inside left area of the cabinet, or someone may have added it to the key ring that has a key for the coin door. If you only have a key to your coin door and not your backbox, then you will need to open the coin door to retrieve the backbox key. If the coin door key is missing you'll have a bit more trouble since you definitely need it in order to get inside the game.
In some cases I've received games that were missing the backbox key. For those games, the first thing I do is see if I can turn the lock by inserting a screwdriver in the key hole. It could be that someone else already realized the problem and fixed it by removing the guts of the lock cylinder after prying the backbox lid off. If a screwdriver won't work, I see if I can use a larger screw driver to pry open the metal lid without damaging stuff, remove the lock cylinder, and using needle-nose pliers, I pull out the pins and their springs from the lock cylinder. These are typically small brass looking inserts with a small spring inside the lock assembly. Of course the preferred way to handle the problem is to simply replace the lock cylinder with a new one once you get the backbox open. I use a lock assembly that matches the front door so that means I often have to replace both lock assemblies.
What if you don't have a coin door key to get inside the game? The easiest method is to use a 1/4" drill bit and drill into the lock where the key is inserted until you can insert a screwdriver to rotate the cylinder. Sometimes you end up drilling far enough into the cylinder that the whole thing gets loose and falls inside the game. No problem, since you can easily open the coin door because there is no longer a lock in the coin door. I've bought so many games that were missing the coin door key that I have the process down to an art by now. I can drill a lockset in less than 10 seconds and open the coin door immediately. I've even gotten the process refined such that I can drill the lockset to a depth that allows me to use any key or screwdriver to rotate the lock and open the door. I have plenty of locksets in my parts bin that it is easy to replace the lock once it's removed. I also buy locksets that are "keyed-alike" so that the same key can be used to open other coin doors and backbox lids as well.
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