Looking at the Taillight Relay Circuit, it looks like your most likely candidates are:

1. Taillight fuse (15 Ampere, fuse number 14). ---- I know, you said you already swapped this thing out.
2. Instrument Panel Junction Box assembly connections / plugs.
3. Taillight relay connections.
4. Taillight relay (Terminals: 1: Coil power; 2: Coil Control -- From Main Body ECU; 3: Contact going to load (fuse); 4: N/C contact going to ground; 5: N/O contact -- Power from same as Coil Power).

John


2007 TCH -- Desert Sand, Bisque -- Original Equip: Leather Seats; NAV; Moonroof; Heated Seats
Completed Mods: Moved the side "HYBRID" emblem/badge to trunk lid; Mud Guards (Colored); Back-up Sensors; Door sill protector; ScanGauge II; Front Bra (Clear LLumar); Window Tinting (40 % Charcoal LLumar); Rear Deck Lid Spoiler
Mods in progress: None
Future mods: Rear bumper appliqué, Sliding Center Armrest