When adding peripherals or troubleshooting a good schematic is needed. I have not been able to locate any board schematics on the web. There is mention of Eagle files, not that they are simplest way to get things, but I can’t even find Eagle files for the 2.6.
The APM2.6 Eagle files used to be on the 3DR Store, but no longer.
This is the best that I can do for you. Maybe someone else will have a link to the schematics.
3DR did build an AMP2.6 version. It was the same as the APM2.5, but without the internal compass.