Pumping at work

Alright so my son will be 2 weeks old tomorrow, I plan on returning to work by the time hes 6 weeks old I only work one day a week at the post office... I know that in will need to pump those days that I work but due to being a mail carrier I will be in a truck usually 5 hours a day, I know I will definitely need to pump at least one time out on my route but due to being in a mail truck and the fact that my breast pump is a plug in only and dont run on batteries I need some suggestions on how to power my pump while I'm not at the office... these trucks are very old so the cigarette lighter outlet thing dont always work very well

This handsome little guy is doing so well with breastfeeding that I dont want this to become such an issue that I get frustrated and stop breastfeeding sooner than I planned so I'm wanting to figure out the solution ahead of time... I already plan on getting freemie cups so that I can pump and work at the same time but I cannot afford to buy a new pump that runs on batteries so anything helps