A whole new RAMPS, Arduino & LCD header later, only to discover that the pin header on the LCD is the wrong way around and I needed to flip the plugs. Sometimes transplanting hardware from one printer to another gives me a damn headache.
It was connected to a MKS Base, which worked, and somewhere I read that the MKS LCD headers are the wrong way around, and I’ve just recalled it. Dammit. That’s about a week wasted on trying to get this working.