snaaake should be the expert....
but I also recently hacked 2 xbox pad...
the shoulder buttons are actually analog buttons... right ??...
(or no ??)
either way... you should be able to find 2 (or 3) big solder point where the whole trigger is attached....
that should be the best place to solder....
but... each circuit board is different... (even if the pad (outside) looks exactly the same...) so... easiest is post a couple of pics.... then guys here might be able to just point out where you can solder....