The fibreglass is tucked on the floor by the rusty car next to the large shack, so grab it and return to Cindy to upgrade your fuel efficiency.
Head for Astor Slough and work your way down the hill to the marker. Talk to Cindy again this time she wants a Redstone Shard to create some paint jobs - this is a non-unique item so there's a good chance you'll already have some by now in which case you can just hand it over, but if not then collect it from the marked area then head back to her to unlock some new decorations for the car.
