Van's has posted a service letter (SL 19-06-27) and associated new wiring diagram for original-model RV-12 owners who wish to add ADS-B Out capability to certain early models equipped with D-180 EFIS and GTX 327 transponder avionics. It leverages Garmin's GDL 82 hardware.
https://www.vansaircraft.com/service-information-and-revisions/sl-19-06-27/
https://www.vansaircraft.com/service-information-and-revisions/sl-19-06-27/