Fundamental rule of thumb:
A. Audio Circuits: Ground shielding only at one end with audio circuits. This elimiates ground loop noise in ICS and speaker feeds. As a rule ground the shielding at the intercom / audio panel / radio end (as applicable) while leaving the other end of a node or lead with the sheild "floating" or not electrically attached to anything on the remote end. That's why headset jacks are installed with insulating washers while their sheilding is terminated unto themselves with a spot of heat shrink.
B. Data Circuits: Ground shielding at both ends with data lines. That's RS232, ARINC 429, High Speed Ethernet or CANBUS.
Shield grounding can be made with enviromental splices to a backshell or to an avionic case as a rule as these components are usually internally and externally grounded and shielded by design. So they are at the same potential as airframe ground. I think the reason the Garmin diagram does not show the "S" in a triangle to designate backshell on the EFIS is because they are showing a generic ARINC 429 EFIS display and not necessarily a Garmin unit. As if to imply follow the grounding instructions of that third party equipment provider. If it were a Garmin specific EFIS you can bet they would put the "S" in the triangle unless it were an unswitched audio circuit for aural warnings.
If you want to have a great training experience in the nuances of shielding, grounding and data lines I highly recommend attending the experimental avionics installation class taught at the AEA headquarters in Lees Summit MO and taught by fellow RV owner Pahan Ranasingha. Very good stuff.
Jim Ivey