Can you locate the area where the leak might be? You're probably looking a removing the siding to look at the damage. Go to where the worst spot is, and remove siding until you've uncovered the entire problem area.
But if you're lucky , all you'll need to remove is trim. (It depends on where the leak is.) Windows and doors are common places for leaks to occur.... not likely that you have a leak in the middle of a large wall - it's probably on a corner or near a window, is it?
First you 've got to find the source, and then uncover everything below it where there's damage.