The AIM (UPC12 - D4) details the wiring diagram for the rear (standard non-RS) tail lights. The AIM also indicates that this rear harness was installed by Fisher Body. Page 16-43 of the Fisher Body manual provides a tail bulb usage chart, and indicates a #67 for the inboard tail lamp, #1156 for the backup, and #1157 for the outer tail lamp. I tried to attach copies of the manuals , but they're too large a file. Hope this info helps.
KK