2000 Buick LeSabre - trunk lights, back up lights dont work

Backup Lamp Circuit Description
Battery positive voltage (B+) is supplied at all times to the REVERSE relay coil through the rear fuse block. Battery positive voltage (B+) is supplied at all times from the rear fuse block NSBU Fuse to the REVERSE relay switch. The powertrain control module (PCM) sends a class 2 message to the rear integration module (RIM) indicating that the gear selector lever is in the REVERSE position. The dash integration module (DIM) sends a class 2 message through power moding that the ignition switch is in the ON position. When these conditions occur, the RIM grounds the REVERSE relay coil. The coil is energized, closing the switch contacts. This supplies battery positive voltage (B+) to the backup lamps. The backup lamps are both permanently grounded, so both backup lamps have power and ground, and then light.

Do you know about class 2 serial data communications between control modules ? Do you know there is a fuse box under the rear seat ?

This is information taken from a Buick service repair manual !

Park Lamps
The dash integration module (DIM) supplies battery positive voltage (B+) to the PRK LP relay coil. When the headlamps or park lamps need to be on or when the ignition switch is in the ON position. Battery positive voltage (B+) is supplied at all times to the PRK LP relay switch. When the park lamps are to be active, the DIM grounds the PRK LP relay coil. The coil is energized, closing the switch contacts. Battery positive voltage (B+) is supplied to the rear fuse block RH PARK LP Fuse, LH PARK LP Fuse, and DIMR Fuse. The LH PARK LP Fuse supplies battery positive voltage (B+) to the LH park lamps. The LH PARK LP Fuse supplies battery positive voltage (B+) to the LH park lamps. The LH and RH park lamps are permanently grounded, so all park lamps have power and ground, and then light. The DIMR Fuse supplies battery positive voltage (B+) to the DIM park lamp input for interior lamps dimming, the license lamps, the underhood lamp, and the cornering lamps. The license lamps are permanently grounded, so both license lamps have power and ground, and then light.

The body control system consists of 3 modules:
• The dash integration module (DIM)
• The instrument panel module (IPM)
• The rear integration module (RIM)
Dash Integration Module (DIM)
Battery voltage is provided from the rear fuse block to the DIM. The module ground is wired to ground. Battery positive sense voltage is provided from the rear fuse block to the DIM. This battery input is referenced to the ground sense input at DIM which is wired to the battery negative terminal and to ground. These sense inputs are physically connected to the battery positive and negative terminals for accurate battery voltage readings. The DIM is wired to the class 2 serial data line. The various DIM input and output circuits are described in the corresponding functional areas as indicated on the DIM electrical schematics.
Instrument Panel Module (IPM)
Battery voltage is provided from the underhood fuse block to the IPM. Ignition voltage is provided from the rear fuse block to the IPM. The voltage inputs are referenced to the module ground. The IPM is wired to the class 2 serial data line. The various IPM input and output circuits are described in the corresponding functional areas as indicated on the IPM electrical schematics.
Rear Integration Module (RIM)
Battery voltage is provided from the rear fuse block to the RIM. Ignition voltage is provided from the rear fuse block to the RIM. These voltage inputs are referenced to module ground. The RIM is wired to the class 2 serial data line. The various RIM input and output circuits are described in the corresponding functional areas as indicated on the RIM electrical schematics.
Rear Integration Module (RIM)
The RIM functions include the following functions:
• Heated seat controls
• Rear defogger control
• Park relay controls
• Various controls for the interior lamps
• Reverse control for the backup lamps
• Rear compartment lid release controls
• Retained Accessory Power (RAP) control
• Ajar switch and tamper switch inputs from the rear compartment lid
• Vehicle leveling
Your best bet would be to take your vehicle to a qualified repair shop ! No offence but this is complex an a factory or professional grade scan tool is needed , could have DTC'S - diagnostic trouble codes stored .
Body Control System Description
The body control system consists of the following 3 modules:
• The dash integration module (DIM), located behind the instrument panel, under the right dash sound insulator.
• The instrument panel module (IPM), located behind the instrument panel, under the left dash sound insulator.
• The rear integration module (RIM), located behind the rear seat back cushion (upper, left).