Is Zakat due on capital invested in the car trade that has decreased due to losses? And is Zakat calculated on the original capital or on what remains after the loss?

The money mentioned has reached the nisab (definition), which is the first condition for the obligation of zakat (definition). The second condition is the passing of a full year (hawl). If a full year has passed on the total amount and zakat has not been paid, then you must pay zakat on all the money. However, if the loss occurred during the year and a full year has only passed on the remaining amount, then zakat is only obligatory on what a full year has passed on.