Most libraries keep reference manuals with wiring diagrams. Thery aren't real
hard to follow. And you can use the copy machine to make a print as long as
it's for personal use. Once you've got the shcematic you should be able to
figure it out. If you can't, SCAN the schematic (high resolution) and ask for
help here. Someone will be able to read it and tell you where to look.
Sometimes, depending on the vehicle, you'll go through a relay before heading
out to your various lighting locations. That's where your problem could be.
Or, sometimes, like in a Toyota truck I worked on last week (same problem, no
tail lights and no courtesy bed light) I found a connector unplugged.
Once you've got a shcematic, and a multimeter you shouldn't have too much
trouble finding your problem.
P.S. Send me your diagram (clean and *.JPG) and I'll blow it up and try to
point you in the right direction.