Turned out the issue was with the black ground wire disconnecting from where it was crimped to the terminal pin in the connector. Upon trying to remove the pin with a removal tool, I tugged the wire and it just came clean out (without the pin!)
Managed to forcibly remove the pin, and have bodged a working solution by re-crimping it onto an extended ground wire and pushing the now mangled pin back into the connector.
Trying to find replacement pins to sort it properly, but struggling to know what to look for. Closest I've found so far are these:
https://www.ebay.co.uk/itm/10-x-Juni...3/140938481475