If you look in the various catalogs (Classic Ind., Rick's 1st Gen, etc..) you will see that they sell Rear Body Light harnesses.
The underdash courtesy plug is part of the rear body harness.
If you car was not originally equipped with the underdash lights, then your original harness will not have the L-shaped connector just above the driver's side kick panel.
Now then, as you can surmise, it would be very easy to wire it up anyway.
Just tap into the wire/s that control the dome light (or sail panel lights if you have deluxe interior). This should get you underdah courtesy lights working.
ps..where did you get your underdash lights?
My car had them, and I have the connector, but no lights.
[This message has been edited by 67LT1 (edited 12-12-2000).]