First, Build a Shed

[PapuaPilot]

The Garmin CAN bus does have a resistor in each end of the bus. It is built into a special termination plug that goes in series onto the components at each end of the bus. Carl, are you sure you have installed the CAN terminations installed properly?

The magnetometer isn't on the CAN bus, so the issue could be that you haven't assigned the RS232 channels.

Here is a copy of my CAN bus and GSU (AD-AHRS) wiring diagrams, maybe they can help.