I want to use a 555 one shot circuit to light a fasten seatbelt light for ten seconds in a car I am building but I have some questions. The input voltage of the diagram I have is a nine volt battery. I want to use twelve volts from the car battery. Is there anything I need to change? Also the output in the diagram is a single red led that isn't very bright. I want to use a bright white led. What do I need to change?

 

A car battery is nearly dead when it is only 12V. When the engine is running and is charging the battery it is about 14.5V.
The datasheet for a 555 shows its maximum allowed supply voltage.
R3 in your circuit sets the LED current and its brightness.

 

I think the timing will be the same with 12 or 14 volts as is was with 9 volts -if you follow the application notes you will see that everything that happens in astable operation is at 1/3 or 2/3 of the power supply voltage.

 

Read the datasheet of the specific 555-timer IC you plan to use and find out the drive current available for your specific LED. If you need higher current you can use a driver transistor.