Hello again,
Yes in automobile applications temperature often deserves reasonable attention so LED characteristic voltage (and thus dynamic resistance) would be a concern.
To set the LM317 up as a current regulator, you leave R2 out and take the load from the ADJ terminal rather than Vout. The output current is then equal to 1.25/R1. For 12.5ma output for example resistor R1 would be made equal to 100 ohms, and the load (LEDs) would connect to the ADJ terminal and ground (where R2 is now). The limits in current are something like 10ma minimum and 1 amp maximum, but the maximum also depends on the power dissipation in the LM317. If the LM317 gets too hot it will cut out and the LEDs will go out, so either proper heat-sinking or low current is the key. Another concern is proper ventilation for the LM317...it has to at least have free air ventilation and shouldnt be all boxed in.