On the subject of LED automotive lights I did some further research;
When attempting to use LED lighting in high reliability, long life and
efficient operation, it appears that some type of regulated driver
circuitry is required which preferably incorporates diagnostics.
Including LED lamps to work with existing vehicle computers is bound
to create some difficulties particularly where the computer only
caters for incandescent lamps which are standard on the vehicle
itself. Trying to mix incandescants and LED's on the same vehicle
appears to be a no-go situation.
A search for LED drivers for automotive applications revealed that
using LED's is not as simple as just wiring a series of LEDs into an
array and connecting them to the vehicle power supply. Some sort of
control module may be needed.
Melexis make an IC specific to this application. However, to include a
diagnostic function seems to be relatively complex.
http://www.melexis.com/relinfofiles/applicationnotemlx1080103.pdf
ST Microelectronics have an IC which does includes a diagnostic
function
http://www.st.com/stonline/products/literature/an/8414.pdf
Analog Devices have the AD8240
http://www.analog.com/en/prod/0,2877,AD8240,00.html
This recent EDN article sheds a little light (pardon the pun) on the
subject of automotive LED lighting and some considerations
http://www.edn.com/article/CA490413.html
