There’s plenty anecdotal evidence to prove prior Iraq/AQ involvement.
1. AQ could not possibly have moved into Iraq as quickly and effectively as it had if there were not an already well known system of safe houses, contacts, money caches, etc and so on in existence. AQ could not possibly have exploited those necessary infrastructure items required to establish itself as quickly as it had unless the leadership of AQ was well briefed prior to the movement. Briefs of this nature can only be conducted face to face.
2. Ansar al Islam was, for all practical purposes, an operational branch of AQ. Same funding sources, same training system based in Afghanistan, same transport networks. Ansar al Islam was protected by the Iraqi army prior to our invasion.
Folks have already forgotten that Saddam sent bio-chem weapons trainers into Afghanistan to Taliban/AQ training sites. You don’t know where to send, when without deep prior contacts.