[aerohead]

The drag which can be streamlined originates behind the car and is a function of the base pressure of the wake.
To lower the drag,the separation point on the cars body must be moved aft,while simultaneously reducing the body cross-section in such a way as to prevent separation,yielding a smaller wake of higher pressure.
Fins can't do this.
They could affect flow onto the rear deck area as with the C-pillar fins on Ferrari's 599XX racer.
Trouble is the cost of the wind tunnel time to dial them in.Which could cost $1,000,000.