Auto Electrical / Radio

HELP!! My son tried to install a stereo system in his truck, about a year ago. He was not successful. The car broke down today, after the engine light came on. The Mellon Street Garage can not fix until the electric wires are repaired. They also connect to the OBD. Any referrals for repair? Anyone know enough about this to take a look? Any suggestions? Son needs truck for work. Thank you for ANY input! Maureen