[n86121]

For the most part the autopilot is self-contained within its own harness.

For example, the 'Computer' gets continuous power from the aircraft through a circuit breaker. The control head switch closes a relay within the 'computer' to power it up.
All inside the 'computer'

I just recently got a full Avidyne suite,
and naturally wanted whatever autopilot capability I could get.

One of the mysteries was where/how to interface to a new IFD 550.

ANSWERS:
The autopilot gets its left/right guidance off the CDI.
It gets its climb descent rate by changing pitch on the control head
It holds altitude with an internal pressure sensor within the 'computer.'

I believe I found the schematic for that in the basic 337 electrical wiring diagrams.
Under, believe it or not, the AUDIO PANEL wiring.

WHAT?

The clue was the original audio panel had a DPDT switch to select 'OMNI 1 or OMNI 2"
It had two wires coming IN (for OMNI 1) that came from the original CDI.
As documented in the audio panel schematic.

I had the tech connect those two wires to the new CDI's similar outputs and PRESTO!
C'est voila!
Left right guidance from the new Avidyne avionics.