I honestly don't know myself.
Try this, get a stamp paper, get your uncle to verify that you've got permission to drive his car, note the engine number etc.
And staple copy of NIC card with the stamp paper. When you get stopped by police, show license, car documents and if they ask for authority letter, just show the stamp paper 
I went to Islamabad airport to pick up my mom once, and the car was my uncles( driver was with me) and i had to make phone calls etc. just to verify it lol 