For 14 to 10 volts in, 6 volts out do the following:

Change R7 to 15K
Change R11 to 68K for about 6.0 volts (you can turn VR1 to set)
Change R5 to 2.7K
Add R16 1.1K in parallel with C5
Add D4 22 volt zener 1N4748 in parallel with C4 (IE across power)
Add D5 22 volt zener 1N4748 from drain to source on Q1 (across the
MOSFET)

From a 10 to 14 volt input it works fine.  There is one catch, though.
When the voltage drops below 9.5 volt the duty goes way too high.
Ideally it would shut off the bulb below 10 volts anyway, as the
battery may be damaged if discharged below that.  So, the following
will dim the bulb way back below 10 in:

connect a NPN transistor Q3 (IE 2N2222 or whatever) up:
  collector to base of Q2
  base to Vout LM317
  emitter to the wiper of VR2

connect a 20K variable resistor VR2 (trim pot) up:
  wiper to emitter of above
  each end through a 10K ohm resistor (thus 2 10K's)

connect the 2 10K's R16 & R17 up:
  one end of each goes to VR2
  the other end of R16 goes to GND
  the other end of R17 goes to power (labeled 5 to 8 volts at the top)

You can set VR2 to the voltage you want the light to get real dim at.
I suggest 10.0 volts.  This circuit will make the light behave
normally above the set voltage, but below it will get very dim.

Willie