Ladytech
The equipment manufacturers' STCs may be applicable for N reg aircraft but most FAA AML STCs are not accepted by EASA so individual approvals are needed for each aircraft. Transponders etc can be installed using approved data from an EASA Part 21J company - nearest equivalent to getting an 8110-3 from a DER. We don't have the FAA Minor change route in Europe where a logbook entry will cover a simple modification.