Avionics How-To Guide

Hello,

I'm interested in learning more about making harnesses and such to make
products like garmin 430/530 talk with a MFD or a EHSI or Auto Pilot.
I'm pretty technical and i'm just interested to see if I can get a
couple products like the above from ebay and just make them talk back
and forth. Can anyone point in the the right direction for any help?
(ie: diagrams, requirements, etc. anything really).

Thanks,

-dr