I think I still don't get it right. I am either using g++ args the wrong way or the includes:

./c++/:

projects/
    cars/
        src/ (Step: #1)
            Car.h
            CarFactory.h
            Car.cpp
            CarFactory.cpp
        bin/ (Step: #2)
            Car.o
            CarFactory.o
        libs/
            #empty
    tut/
        src/ (Step: #3)
            Tut.cpp
        libs/
            #empty
        bin/
            Tut
libs/
    libcars.so

I did the following:

Step #1:

g++ -fPIC -c *.cpp 
mv *.cpp ../bin/    

Step #2:

g++ -shared -fPIC -o libcars.so *.o

Step #3:

g++ -c Tut.cpp -o ../bin/Tut -L../../../libs/ -lcars
Tut.cpp:3:24: fatal error: CarFactory.h: No such file or directory
compilation terminated.

Tut.cpp:

#include <vector>
#include <iostream>
#include "CarFactory.h"
#include "Car.h"


class Tut {
private:
    std::vector<cars::Car> cars;

public:
    void runTut(char method) {
        this->cars = this->orderCars();
        this->testDrive(this->cars);
        }
    }

    void orderCars() {
        std::cout << "Ordering cars.. " << std::endl;
        cars::CarFactory cf;
        int carsToBeBuilt = 10;
        for (int i = 0; i < carsToBeBuilt; i++) {
            cars::Car car = cf.buildCar();
            this->cars.push_back(car);
        }
    }

    void testDrive() {
        for (std::vector<cars::Car>::iterator it = this->cars.begin(); it != this->cars.end(); ++it) {
            std::cout << " driving: " << it->cars << std::endl;                   //trying to print the car's mem address as car id.
        }
    }

};

int main(int argc, char* argv[]) {
    Tut t;
    t.runTut();
    return 0;
}

I guess, I do not have to put a different path into the #include because I tell the compiler where to find the lib that contains the needed objects. It's not a wrong path, it doesn't even compile when the lib is in the same directory.