Hmmm...this seems very strange. There is no reference to a ground or other connection in the wiring diagrams and the actual locking actuator is pneumatic not electric anyway. On mine the lock works irrespective of whether the flap is open/closed or if the cap is on or off. There is also no reference to a connection on the flap in the removal instructions. The wire might be there to discharge static to prevent a spark when the cap is replaced. Perhaps the US versions are different to the UK. Have yo got a picture?