I think what may be contributing to some confusion is that the '86 service manual has a mistake and two omissions regarding the pinout of C500 for the rear brake lights. Regardless, if you installed a fastback harness, forward of the engine firewall starting at C500, you should have been able to plug the notchie rear half of the harness directly into the C500 and it should have worked.
To reset everything where it was, unplug C500 and check the following:
1. the front chassis half (new fastback harness) should have a white wire at G7, and a white wire at H8;
2. If there is no white wire at G7, but a blue one at J9, then pull the plastic pin-retaining clip out of the side of C500 (looks like a fork with many tines when removed), then use a bobby pin to release the metal retaining tang on pin J9, and reinsert it into G7;
3. on the rear chassis half of C500 (originally your notchie harness), make sure there is a white wire in pin G7 and one in H8.
Once that's done, everything should work when plugged in. Post what you find if things aren't configured as above.
Edit: erased some extraneous stuff)
[This message has been edited by Bloozberry (edited 02-05-2011).]