there is probably a crack on the rupper protection over the cable(so water goes in and rust appears),so i would suggest you to remove it from the caliper,and then,shoot some spray grease between the cable and the protection rubber tube,then pull and push in to make the grease go all the way in it...you should be good...
anyways if you need to change the cable,wich is absolutely normal after 5-6 years of using it,it cost around 50$ at any autoparts place,or even at mazda,and about a half hour to change,so if i was you,i d replace it right away,your safety depends on it!!!
good luck buddy!!!