When you are already setting the naked wires, why don't you just use LM2805 and connect directly instead of having two wasteful circuits (a car charger which probably has an LM2805 in it, and then another non-switched, highly inefficient dropper in the Nokia power-to-USB converter). Your external battery will last longer, there will be only one heat source (LM2805, and if you want more efficiency you can go for a switched voltage dropper), and you won't need 5 different points of connection.

Or you can at least avoid the car charger alltogether, Nokia's converter will do the job by itself, although it might melt if connected to 12V (depends how much current external battery can pull out), but you can offset that to a normal wall-charger output with a simple zener diode before it...