Sorry for the delay in replying. I just got home from a month long hike.

The transistor: I can't recall whether I used a 2N3904 or 2N2222. Either (or probably any other NPN transistor) will work the current is so low.

            |--R2 --LED1 --LED2 --LED3 ---Q1---------|
12V+ --|                                             |           GND
            |-------R1 ------------------------|--LDR ---|

R1 -- 47K ohms (may have to adjust for the LDR you are using)
R2 -- 120 ohms (for three white LEDs in series. May have to adjust a bit to get the current you want.)
LEDs -- generic white LEDs
Q1 -- 2N3904 or 2N2222 (Collector to LED string, Base to R1 and LDR, Emitter to GND)

Here's a better photo of the circuit. Keep in mind that the black wire is the positive and the red wire is the negative. (That's part of what messed me up in the first place. I wrongly assumed the red wire was positive. Check your plug to see which wire is positive.)