Quoted from EMpins16:When the game first is turned on the 2nd player display works , but only that one . When a credit is added it no longer works .
With Dolly Parton, when you start a single player game the player 2, 3 and 4 displays go blank and this is normal. Press the start button again for a two player game and the player 2 display should come to life showing '00'.
Anyway it sounds like you have some connector issues. It will help your cause if you post some pictures of the boards in the backbox.