The only study I can find on this is a 1997 Australian one [2] which examined three years worth of crash data (1989-1992) from Victoria, looking at the 606 people with some kind of head or neck injury. After looking at various potential methods of injury reduction they determined how much each would be expected to help over a baseline of just seat-belts and front airbags. They harm reductions (taking into account both injury frequency and severity) of:

Soft-shell bicycle style helmet	12.0%
Side curtain airbag	3.5%
Roof padding	2.4%
Seat belt pretensioning	1.9%
Improved side windows	1.7%
Rail and pillar padding	1.6%
Removing the passenger front airbag	-1.5% [3]
Removing the driver front airbag	-10.8%

(These are all as percentages of the total harm to occupants.)

This was in Australia in 1997; many of these alternate improvements were standard on cars in the US then and even more are now. Imagine side curtain airbags, padded interior, pretensioned seat-belts, and the fancy window-glass the paper suggested adopting all included now and all perfectly complementary with each other but perfectly anti-complementary with helmets. Then they add up to an 11.1% harm reduction and leave only a 0.9% harm reduction for helmets. At the other extreme, if helmets are entirely complementary with whatever cars have now, then the full 12% harm reduction stands and helmets are about as helpful as front airbags.

I'd like to see something looking at the benefits now, when combined with current car-safety systems, but I can't find anything. I also would like to get am estimate of what would happen to my 1 in 5000 chance of death from road traffic accidents if I started wearing a helmet. I think it becomes roughly 1 in 5000/(1-X) where X is the harm reduction and is somewhere between the 0.9% and 12% above. (So between 1 in 5045 and 1 in 5681.) Is this enough to mean I should start wearing a helmet in the car when its convenient?

[1] I have about a 1:5000/year chance of death from road traffic accidents. (USA, male, age 25-34: 21,223,534 people; 4343 such deaths in 2009. Data from here which is ugly and confusing but still far more usable than the CDC's official system which involves hundreds of large unsearchable monospace-tabulated PDFs.)

[2] Prevention of Head Injuries to Car Occupants: An Investigation of Interior Padding Options

[3] The baseline for all the figures is cars with airbags and seatbelts. If we remove the airbags harm goes up, so these numbers are negative. For comparison to other interventions, looking at the magnitude is reasonable.