Header based on client code in document

I have a VBScipt that creates letters in Word templates and merges customer info with bookmakers from an Access DB. Currently the letter templates all use the same header I need to change this to choose a header based on a client code inserted from the one of the bookmarks. I'm all set on the process to actually add the header just not sure how to pick the correct one. So basically I am looking for if code if this use this use this header. Haven't had any luck yet any input would be appreciated.Thanks.