The magnetometer is a little gadget that measures the strength and direction of the local magnetic field, which allows the AHRS units to figure out their current heading, among other things. If you read the installation requirements literally you might think that there's nowhere in the airplane you can mount it to meet every one of the requirements, but in the RV-7 a good spot is the F-714 aft deck, just ahead of the horizontal stabilizer. Garmin makes a mounting plate you can use for this, although anyone who has progressed this far ought to be able to knock one together from scrap aluminum in no time at all.
I happened to have one laying around, so after some careful alignment I drilled the fastener holes through the aft deck:
The magnetometer mounts into this combination nut ring and alignment fixture, which I riveted to the plate with AN426AD5 rivets. A rivet that big is way overkill for such a lightweight piece, but that's the size of the countersink you're given so that's what you've got to use.
I bought some brass screws and locknuts from McMaster-Carr, and used them to attach the plate to the aft deck. Brass because you want to avoid putting ferrous material (e.g. steel screws) next to your magnetometer. I could have used rivets here, but I wanted this plate to be removable in case I ever need to reach my hand down in there to do maintenance work. Also I really didn't want to crawl back into the tail again to set the rivets.
Test-fitting the magnetometer to determine where to terminate the wires:
I put a tie wrap anchor on the aft face of the F-709 bulkhead and used it to secure the the connector for the magnetometer wiring:
Wider shot of the aft fuselage with magnetometer installed:
Happily, the magnetometer came up and started talking on the first try:
