There's only one Shell Pump in G-10 Markaz which is on a double-road. So after crossing the pump on the left side, just a bit more forward and you'll see car shops to your left and right (you'd have to turn left to see better shops). After you turn left, on that small load, there would be another pump known as LILA CNG. Now as soon as you cross that, park your car somewhere and start checking shops from over there and keep going forward. There's a decoration shop over there after the Lila CNG pump. The guy usually running it is Mazhar.
You can ask him or his younger brother (whoever is over there) for those wipers.
If F-10 is closer, there's a shop known as Naeem Auto's (the guy's pretty decent in his dealings, you can try that too if you want).
My Suggestion, G-10 is usually cheaper as compared to F-10 but whatever you do just roam the market and find the best price of the product before buying it (just to be sure).