Conclusion:- I would forget about capacitors and have a small rechargeable
battery in the circuit.
Then, the battery on the 5V pin would backfeed power when VIN is lost
You use diodes to prevent reverse current flow. I am not sure what you are concerned about backfeeding. A linear voltage regulator is quite happy having power applied to it's output with nothing going in and the series diode in the power jack would prevent any further loss to the rest of the car circuit.
How do car manufacturers do it? ...... these continually waste power through the drop
Yes they do. They rely on having a huge capacity battery, in relationship to the current draw when off.