Make a circuit similar to following:

+Power O----R1---L1---D1---*---O + to 7805
                           |
                           C1
                           |
-Power O---R2----L2--------*---O - to 7805

Make sure your circuit does not touch the car ground anywhere. C1 is your
existing 22uF. R1 and R2 are 10 ohms each and L1 and L2 you make by
winding bifilar wire on a small toroid. It won't work if the input voltage
drops below 8V with a 7805 regulator.

Peter

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