Yes ,Indian citizen(common man) can definitely fix a national flag on the car. The rules have been changed accordingly sometimes back.
But it is nessesary by rules to maintain following things about it
It must be clean all the time, should not get mutiliated or torn, failure to maintain it amounts to insulting of the nation.
I refrained myself from fixing a flag to my car because It is common for the bus travelers to spit on bonnet of a sideby moving car.
And you never know, what street children will do to it , in an attempt to take it away.
For IPS ,IAS, politicos , it is left hand's index finger game to bring the errant bus travelers, children , and us also in to the books immidietly by virtue of the red beacon, in case of insult of the flag.
So, it is better to express the patriotism on the dashboard IMO