Say I hardwire positive and negative wires into the car's 12V system. How do I step it down to 5V safely and reliably? Resistor(s)? Any ready-made solutions available?

Thanks

 

There are many off-the-shelf car socket to USB chargers out there, which would take 12v in and put out 5v,
that you could adapt for your needs.
This adapter, however, looks like exactly what you want, though of unknown quality.
If you need more than a couple amps of current, though, you'll need a more robust solution.

 

I broke open a USB cigarette lighter charger, soldered 2 wires to the input for 12Vcd & connected to an exsisting 12Vdc source. Heat srunk the whole deal leaving about a 1ft. tail. I keep it looped & stuffed in the net pocket on the passenger side of the console. Charges the phone no problem. The adapter oth linked to looks good too.

 

Thanks guys

 

If you haven't done this yet, then get a regulator.
http://www.radioshack.com/product/in...ductId=2062599