have you tried recoding the key to the csm (central security module). you have re-set it by unplugging it. procedure in the handbook.
key in ignition, turn to position 1
turn key from position 1 to 2 and back 4 times.
ie 1-2-1-2-1-2-1-2-1
led for alarm should light up constant.
press and hold one button on remote (doesn't matter whitch one) - leave key in ignition on 1 by the way.
led should start to flash
hold button down until flashing stops ie light stays on
release button
start engine
switch off ignition and remove key
test
if it doesnt work unplug module for 2 mins and try procedure again
if still don't work book into ford for wds2000 test