A joint return does not show individual income. It's "joint".
I seriously doubt you'll be able to get any useful information from this public user-to-user forum, as your question pertains to what is a foreign taxing authority when viewed from the perspective of this forum. I can only suggest you seek the services of a CPA or tax attorney in your local area, and good luck finding one that has the first clue about foreign taxing authorities. If you're close to the Embassy or if there's a German consulate office close by, perhaps they can assist with this.
