From Bruce Roe (Feb 27 2002):

If that light just blips off and back on, your heater was 
continuously powered, just not quite enough for operation.  
If your warmup time is over 50 seconds, you need to put 
some of those parallel resistors* across the 1 ohm power 
resistor.  Another way to check this is observe the voltage
across the 1 ohm during warmup to operation.  That 
voltage will hold at 1.25 volts during warmup, then should 
drop back 5 to 20 % (below 1.19 volt) within 30 seconds 
of the LED coming on.  If it is too close to 1.25 volts, you
need more shunting resistors.  Maybe we ought to 
add this to the testing procedures.

If the above works, but the LED still won't stay on, you 
need a bit more voltage.  If you can't  get it from your 
present source, try a pair of wires directly to the 
battery, or the alternator case and output stud.  

There is an outside chance a sensor might be found 
exceeding the regulator current capacity;  a 39 ohm 1/2
watt resistor in parallel with the regulator ought to fix 
that by adding 75 ma.  Maybe a couple will be needed.
I don't really expect this to happen.



===== addendum 11/09/2004 GAP ========
parallel resistors* : the parts kits included several 1/4W 
15 ohm resistors which can be added in parallel with R4 (the 
5W 1 ohm monster) to decrease the warmup time. About three 
of these 15 ohm resistors is typical.