Where are all the ground wires located on a 2008 chevy cobalt?

It wouldn't be a ground problem If it were the whole instrument cluster would be dead . Checking the BCM an the IPC for DTC'S - diagnostic trouble codes is the proper way to diagnose this type problem . Not worrying about grounds .
Odometer
The vehicle odometer is calculated and stored electronically in the body control module (BCM) and is sent via the serial data circuit to the instrument panel cluster (IPC) for display. The BCM calculates and stores the mileage based on the vehicle speed serial data information from the PCM/ECM. The IPC displays the season odometer information from the BCM in the driver information center (DIC). The odometer will display error if the IPC detects an invalid odometer data from the BCM. After an invalid odometer data is received from the BCM, the IPC will display the last known odometer reading for 1.5 seconds before error is displayed in the DIC.
Speedometer
The instrument panel cluster (IPC) displays the vehicle speed based on serial data message from the body control module (BCM). The BCM receives vehicle speed data from the PCM/ECM via the serial data circuit. The speedometer will default to 0 km/h (0 mph) if:
• The PCM/ECM detects a malfunction in the vehicle speed sensor circuit.
• The BCM detects a loss of serial data communications with the PCM/ECM.
• The IPC detects a loss of serial data communications with the BCM.

Your best bet would be to take it to the dealer !