Can I ask, what's the ohm reading on the resistor you used ? as most "other" cars use 3.3ohm 1.25volt (not to be confused with a 3.3k)( I think?)), although as I said my automotive electrical knowledge only extends to fitting stereos, spotlights and associated wiring etc.

Maplins do a range of resistors that aren't expensive, so maybe worth picking up a few with higher values ? and trying them.