Taillights won't work

I would check the assembly next. The assembly is the plastic thingy that you stick the bulbs into, after you pull the bulb out you should see two clean and silver shiny metal tabs. I f the tabs are pushed down, rusty or bent in a direction away from where it needs to connect to the bulb, there is a loss of electrical connection and the bulb (tail light) wont work. It is possible that at the age of your car the assembly may have severely rusted tabs and needs a new assembly to make good connections with the bulbs again. Typically if bulbs and fuses are good like you say it only leaves bad grounds or connections, pull a non working tail light bulb out and check to make sure the metal tabs are in condition to make a good connections, if they are then check your grounds because that is the only thing left but usually those tabs are it.