Assuming it's wired correctly and unless you've got a stray strand of wire shorting two lugs and making a connection, then chances are it's the switch itself. They're a break not make switch, as in the default is both contacts On in the middle position and then it breaks the opposite contact in the up or down position. So, in the neck position it breaks the bridge contact and vice versa. In this case it may not be 'breaking' on one side. If you can confirm it is the switch then you can try prying it open to fix it, it's been done before, but it's far easier to just replace it.