Unfortunately, this is one of those topics that can't be discussed in these forums. See, MAMEDev, for legal purposes, frowns on coin door usage and being that this is a MAME friendly site, I am guessing this post will be shot to POST HELL in no time flat.
Now that said, keep in mind that a coin door simply has a microswitch in the path of the coin that gets hit causing a credit to be added. With that in mind, take a look at your coin door and work your way back from the switch.