If I'm at the carPC, I can type "//192.168.0.101/" (the IP address of my laptop) and then I get a username/password prompt. I enter the correct username and password, and I have access on my carPC to the shared folders of my laptop.
If I'm at the laptop, I can type "//192.168.0.102" (the IP address of my carPC) and then I get a username/password prompt. I cannot enter any username/password combination to give me access to the carPC's shared folders from my laptop.
This means I can't load new files onto the carPC from my laptop. I actually have to get out the keyboard and mouse and use the carPC to load new files.