Use other fields to pull values from

I have a big problem. Im trying to autocomplete a image alt field that gets its values from a rewrite rule... Not good at all. I want to be able to use a Taxonomy field to get the values to use for autocomplete on my image alt field. But they do not have the same name. So i had to go into the php code and remove the code where it only showed fields with same name. Once i slected the Tags field i want to use, it still doesnt pull any values from my Tags field...