I’m in the process of converting my tailights on my car to LEDs.  I have 12 LEDs wired in parallel.  I was wondering what resistor(s) I need to hook these LEDs to a 12V connection in my car.  The LEDs I used were part # RL5-R3545 from superbrightleds.com (628 nm RED 3500 mcd/45 degrees).  How much resistance should I use for this connection and where?  I’ve attached a picture to help clarify.  I appreciate any help.

No, that will not work.  You want your LEDs in series, not parallel. Here is a diagram depicting both.  Top is series, bottom is parallel.

Make sure you also decouple(put a capacitor between +12 and Ground) to prevent electrical noise.  I don’t think the power supplies in cars are very stable.  You didn’t provide the specs of the LED, so I can’t tell you what resistor.  Look them up and plug them in here:

http://led.linear1.org/led.wiz