For next time:
Using the car lights to help auto focus is one option.
Another handy thing is to have a small LED torch in the camera bag for such occasions.
If all else fails (ie its too dark to even see if its in focus or not - like it did for me when I took the photo below)
just turn auto focus off and then manually set the focus to just before infinity
This reminds me - I did a couple of 30seccond photos the other night actually (during that lightning storm) - I should really see how they came out!