The only problem i see with the use of the 7805 series voltage regulator is its min and max operating tempratures. Min of 0 deg C and max of 85 deg C is not suitable for an automotive environment. You need -40 deg C to +125 deg C.

Try a LM2937ET-5.0 or, a LM2954AIT-5.0 if you require an output voltage tolorence of 1%. You will need i/p and o/p caps with both parts or you will see near supply voltages at the output. I have used everything from 0.1uf to 330uf at the output on both parts and it always works fine. Remember to spec your input cap with a minimum of 35v.

Here is an example of what i am using on a tyre pressure control system for a semi:-

LM2954AIT-5.0
AAvid heatsink 592502B034
input cap - 220uF 50v Panasonic EEVFK21H221P
output cap - 330uF 25v Panasonic EEVFK1E331P

All available from www.digikey.com

Probably overkill but safe as houses.