I've got a 2005 S-Type, with the commonly broken gas door actuator. Purchased a replacement (XR857532, genuine Jaguar part). However, it isn't working. If I have someone press and hold the button, I'm getting 12.7v at the power connector. If I bench test the actuator, I can get it to move by applying voltage. However, when it's plugged in to the power connector, I get nothing. The only thing I can think of is that possibly the connector isn't making good enough contact when the two are connected together, but I don't think I can just purchase a new connector for the wires. Neither the original actuator or the new one contract when connected to the connector and the gas door button is pressed.

 

You can get the connectors separately
Here for example http://www.wiringproducts.com/ford-connectors-sockets

 

It may or may not be the connector. With the ignition switch on, you have battery voltage at the power connector all the time. The button switches the ground on, so completing the circuit and moving the actuator.

So there could be a problem anywhere on the ground side.