1980 300td lower wire harness

The lower wire harness going to ac,pressure switch,starter,ac,and alternator is burnt. Currently the vehicle is at a shop with whom I have been doing business for over 10 years. The mechanic suggested I try to find a used harness and have it disconnected from the engine side and cut about one foot inside the firewall. From there he said it would be the best and least labor intensive. The problem I am having is finding a vehicle that old without the harness being cut at the fender. According to MB only that vehicle and production nos. between 165,000 and 211,000 in the sedan are the ones to be looking for. I have been looking without luck. Am I on the right track? Do you have any suggestions? Thank you