How to enable voice-activation of S Voice when in car? I am only interested in using S Voice with voice activation (i.e. "Hi Galaxy!") when driving.

Is there any way to activate S Voice when I plug my Samsung Galaxy S3 into my car's charger?
Take a look at the app Tasker. Also the free app Llama may also be able to do this.

Tasker and Llama automate tasks on your phone. So you can have them say IF this condition (car charger plugged in) THEN do something (activate SVoice).