If your head gasket is gone then you're best getting a stealer or specialist to do it IMHO. Are you overheating, losing coolant, getting a pressurised system, got gunk on the oil filler etc? If so, then it sounds like the gasket has gone.
As for the harness, you can check by removing the connectors at the ECU end (in the box where the jack sits). It will be obvious if you have a problem. If you do, then it's a very simple job to replace the injector harness (remove engine cover, remove camshaft cover swap out harness and refit everything), no more than 30 mins work.