go to harbor freight and buy a cheap 5 dollar
ohm meter.... set it to volts... touch one wire
and with the other wire from the ohm meter touch the
car chassis... on one wire it will read voltage and the other will
read nothing.... the wire with voltage is postive...

you can check your self by setting it to ohms and the wire
that had no voltage will cause the meter to jump when touched
to the car chassis...