A little engine that doesn't stop
 

I have a Pontiac Firebird from 1992 (150hp, 3.1, V6) and when you open the right door you can see there a little engine (you can touch it with your hand) and when the main one is completely off, this little engine is still running.

So what's for that little engine and how to stop it? It only stops when I turn the ignition key till the position where I can hear the radio and I switch on this last one. Otherwise it's neverending.

Please, I need help!!!

Thank you