When does the sound get funky and thin? When you turn the knob in a particular direction or all the time?
The wiring diagram should be that of any other similarly-cofigured guitar. I would first make sure the pickup is OK. Try unsoldering the pickup from the controls, connecting the leads to alligator clips and clipping the other end of the clips to the tip and sleeve of a guitar cord. Plug into the amp. If it sounds OK you've ruled out the pickup as the problem.
To add in the tone popt, I would use something like this:
http://www.seymourduncan.com/support/wi ... =1phat1v1t
As a diagram of a 1 pickup, 1 volume, 1 tone guitar. If there's an old tone capacitor in there now you might want to consider not using it at first. If the guitar works OK without the cap, try including it. If it sounds bad, replace the cap.
Hopefully this helps,
-Scott