Kev,
As stated above...I did the same routine that it sounds like you are doing. I chased everything I could think of to no avail. The good part is that when I finish up with the rear end wiring this week I will have all good ground connections to work with as they are all clean, bright and shiny!
The melted wires I found were making contact with each other under the dash which I suspect is what was causing my "bleed over".
If it looks like the PO has been under your dash making questionable connections I would either remove your current harness and repair it properly (i.e. cut, crimp, solder and heat shrink insulation) or replace the whole thing. I chose to replace it since the repair job would have taken just as much time and I still would have ended up with a 40+ year old wiring harness.
By the way, if you haven't been working on the wiring harness lately and everything was working before then it definitely sounds like you have a wire crossed somewhere due to age, corosion or failed insulation. When looking at the dual filament bulbs in the rear the "dim" filament should be your tail lights, and the "bright" filament should be the turn signal. Your description above leads me to believe that somehow you are getting voltage from the running light circuit into the turn signal circuit, hence the "bright" tial light that stays on all the time with the park lamps on.
