The radio uses a harness and has a Sigtronics 200S dual intercom installed.
Harness is here...
http://www.aircraftspruce.com/catalo...201harness.php
Sigtronics intercom is here...
http://www.sigtronics.com/pdf/air_pd...00S_E_INST.pdf
Where and how do you suggest wiring in the test microphone?
What type of microphone is suitable?

Thanks
RMM

It receives distant aircraft transmissions OK? Then the antenna is likely connected.
Wire a test microphone directly to the rear inputs of the radio and try that.

That does not work then send it off to Becker for repair.