ATU100 and 200 share the same harness, the 300 is a different connector, Deere offers an adapter harness to reuse the main harness and connect it to the 300. Assuming the combine has the black and grey connectors for the brown box hanging out of the headliner, from there you need an adapter harness that connects to these connectors and has the 26 pin square connector for the GS2 or GS3 displays, there is a Y harness that connects into the black and grey connectors that goes to the ATU