This can be simple if you have a schematic....
Look at the turn signal area of the schematic. It should show that there is a plug that mates the turn signal harness to the underdash harness.
Locate that plug, it should be affixed to, or near the steering column. Unplug it.
I'm assuming you've check out the flasher, or replaced it.
1. Find the three wires that come from the turn signal stalk. Now one should be common, and two should be right and left.
2. Put an ohm meter from common to right and turn the stalk to right.
3. The ohm meter should read zero ohms. If not, the switch is bad.
4. Repeat steps 2 - 3 with the the stalk to the left and the ohm meter on the left wire.
5. If this checks out, then the problem is in the dash wiring.
6. On the underdash harness find the 12 volt common wire that feeds the steering column turn signal switch.
7. Find the left and right return lines from the switch.
8. Put a jumper from the 12 volt common feed to the Right wire. The right turn signals should come on.
9. Do the same for the left..
You've now cut the problem in half, and tested each half. Let us know what you find.