I've got hold of a spare key and fob for my2004 Forester 2.5 XT with the 1 button Sigma factory alarm. I thought I'd be able to take it apart and put in a fresh blade to be cut for my car, but I can't figure out how! Any help would be welcomed; I've programmed the fob to the car.

 

Image below of the key:

 

 

 

 

Not super simple, new key will also incorporate transponder chip cloned from current key which will need go into the fob along with cut down blank, good helpful auto locksmith will do it or you can chop it about yourself, transponder chip is very small.

I guess another option would be to hacksaw the blade off and just use it as a remote fob alongside a freshly cut key/transponder. Would the two transponders then cause technological terror on the car or would it just recognise the one transponder and ignore the other?

Best without 2 transponder chips so ideally needs removing, tweaking another blank in it should be possible with bit of innovation but indeed it could be used like a fob.

I just can't figure how to get the existing key out; looks like its molded in. Where abouts does the transponder sit? Is it in the metal key itself?

Not 100% sure, have swapped remote boards in them & the responder chip is somewhere in housing, they quite small.

Have a word with some auto locksmiths & also see if subaru can confirm how new key comes, whether blank shank or complete housing !

A new key with transponder is about £20 or a little over, it may work fine with 2 transponders & still pick up the proper one but it also may be temperamental.

another option (but fiddly) if key is moulded in is cutting it so a stub sticks out then cut down new key to overlap & drill 2 holes for very small machines screws & tap them or even rivets, then tidy the join & add 2 or 3 layers of heat shrink tube, key overall would become a little longer but could be neat result with some patience, done similar to keys for workshop door that has custom lock.