Try replacing the brake/tail light bulbs, the marker light bulbs, and the dash light bulbs. These bulbs all go out at some time or another; and often are not noticed until they are all not working. The other thing that causes rear lights to not work is poor ground connections. The brake/tail light assemblies are usually grounded with a ground wire from each side to a bolt in the frame or body. If these ground wires get broken or damaged, or the connections become corroded; it will stop the lights from working. On some vehicles; the light assemblies are grounded through the mounting bolts. This type of grounding is liable to not work if the body has been repainted, or if rust has set in. In those cases; it is necesary to either scrape the paint off the body under the light assembly mounting points; or to install a dedicated ground wire from the light assemblies all the way up to a known good ground point. In badly corroded bodies; that wire may need to extend all the way up to the battery ground terminal. There is also a ground wire from the firewall to the engine; and often another ground wire from a bolt in the fender to the battery negative terminal. If these auxiliary ground wires get broken or are removed; it will mess up the lights all over the vehicle. So check whether these wires are on the vehicle; and replace them if they are not there.
Fuses will often go open in spots which cannot be seen. For that reason; it is not reliable to check fuses by just looking at them. The only sure way to test a fuse is either with an ohmmeter, or by substituting a new or known good fuse of the same amperage in its place.