Even though my E91 is fully functional, I will not disassemble it to check the voltage while turning off the radio. With the original head unit, amplifiers, etc., I also have.
Is the voltage still not being measured when the radio is turned on? After you installed the aftermarket head unit, did you connect this wire to the amp on signal?
When you fixed the broken wires before, what colour were they and how did you do it? So, you fixed the four wires inside the gaiter using solder or crimps, or you replaced their entire length so the joins were on opposite sides?