Start from the RCA connectors and work back.
1. Switch the RCA's going into the pre-amp. If the dead channel now becomes the left, the problem is the cabling. Replace the RCA cabling.
2. Make sure the head-shell pins going into the tonearm are clean and make a snug fit when inserted. I had a similar problem with a Thorens TP60 headshell and discovered that the pins were not inserted tight enough.
3. Make sure the four head-shell wires are in good condition and are soldered properly at all eight locations (four connecting to the cart and four connecting to the head-shell).
4. Switch the left/right cartridge connections. If after you switch the left/right connections the left speaker is dead, then it's the cartridge.
If after you have tried the above the problem still persists, then there's probably a bad connection in the tonearm wiring.
Good luck.