G'evening hc......I am as wise as you are on this issue...In fact I Do Not have a smart phone or a tablet and often get lost when discussing matters outside of Computers/Laptops.
Do you have any on screen indication that your Nexus is being charged when plugged into your car battery socket.? If Not then there is obviously a lack of connection, which suggests a faulty cable/charger. On the charger which I suggested the cable will take a heavier loading than that which you have. However the one you have should be adequate to charge your Nexus. It is the voltage which is important not the amperage and I believe you require 5volts.
In view of the fact that the charger I suggested is very reasonable to purchase, I would suggest that you have a go with a new item. If it doesn't work you could ask me to refund you the cost. :-)
Hans.