This is a situation where you need to pull the door panels and start checking wires and such. If you don't have a volt meter, a test light and jumper wires will help. One thing to start with is see if your locks are gummed up. The grease looses some of its lighter fluid over time and gets stiff. Some penetrating oil can free this stuff up and sometimes that is all it needs for the power locks to work. Also you can get broken wires at the boot between doors.