Jason, Hard to diagnose these kind of problems remotely – do you have access to a multimeter? If so, you can check through each connection from the drivers to the relevant side motor connection step by step.
One thing you do have to be careful of when doing conversions, is not breaking one of the wires connected to the keeper plate, as it’s difficult to get enough length on them when removing the plate. Maybe worth checking these (I know it’s a pain getting in there!).
Have you removed the DCC blanking plugs for any reason? Again, maybe worth checking they’re still plugged fully home.
Hope that helps.