RST Engineering wrote:

Where is the regulation on what it means for an individual to be "certified"
to work on avionics or "certified" to work on instruments?

The tech can be FAA certified as an "A" of the A&P, ie Airframe, the
work signoff comes from the Repair Station itself who oversees/approves
it...

FAA Certified aviation repair stations:
FAA Air Agency Certificate Instruments are divided into 4 Classes, 1
(Mechanical), 2 (Electrical), 3 (Gyroscopic), & 4 (Electronic)
Accessory Class 2 = Electrical Accessory Class 3 = Electronic
Radio Class 1: Communications Equipment
Radio Class 2: Navigational Equipment
There is also a Limited Airframe rating for the Repair Station.

JAA has JAR Acceptance Certificates

All the beer you can drink at Oshkosh (at one sitting) for the right answer.

So the short answer is, there isn't one... Will it have a Root in it?