On Friday, September 6, 2019 at 8:16:50 AM UTC-7, wrote:
I’ve purchased three harnesses over the last few years. One made by Goddard was by far the nicest unless you try and get an original factory replacement.
The factory replacement harnesses for these older radios aren’t readily available, so you might end up with a newer model radio harness and have to
dead end a number of wires in the harness for functions the older radio doesn’t have.
The Goddard harness I purchased was well made, Tefvel wiring, each wire and function labeled, appropriate ends on the wiring for shielding, etc. it’s the only way to go as far as I’m concerned, especially with an older but quite functional radio!
And I’m sure any of the soaring supply distributors would appreciate it as well.
Regards, Mark

For 90% of pilots, that's the way to go: Buy a harness for the new radio.
And for those thinking about upgrades, the stuff from 30 years ago may not be compatible with new models but some intermediate models may have the same wiring.
I've recently replaced a FSG-71M with an FSG-2T... Plug and Play.
Also replaced an AR4201 with an AR6201... Plug and Play.
Removed another FSG-2T, replaced with XCOM, a bit of work and a microphone change.
Later replaced the XCOM with a TY91. Ordered a harness with the TY91.
Consider the compatibility of microphones in the upgrade. Some radios do not have inputs or preamps for different microphone types. One reason for changes to the wiring harness of newer radios.
The newer ones may have auxiliary audio inputs. Some have separate microphone and PTT connections for each seat. More attention may have been paid to grounding schemes.
There are different pin counts and locking types for D-Sub connectors.
Pretty much replacing anything that doesn't fit in a 57mm cutout is going to take some work. But it'll be worth it!
Jim