How to re-start the cars own amplifier / antenna
Many late model cars will have their own amplifier or antenna amplifier which will more often than not be powered from the original head unit. With any aftermarket unit these will need to be re-powered. The methodology is quite simple.
1) With the old stereo unit connected probe and isolate each wire which is powered when the unit is on. Then disconnect the plug from the back of the unit and re-test these wires. The wires which no longer have power going to them when disconnected from the head unit need to be re-powered. These power wires could be running power to the amplifier or the antenna amplifier and need to have 12V re-connected to them.
2) For the external amp: if you can locate the amp, then using the original unit connected probe each input and isolate the power wire. When installing a new unit then again make sure this wire is powered