Industrial Turnkey Automotiv project. Interfaces between vendors

A time tested and easy tool for that is an NxN (N-squared) interface diagram. A matrix is created which can be created at the vendor level, system level, part level or whatever makes sense in your case. Flow of data is depicted in a clockwise fashion (e.g. Vendor A needs to give Vendor B power requirements). Each interface gets a numerical identifier, and a spreadsheet is created which includes: provider, receiver, deliverable (e.g. CAD model), and due date.