Why people even disconnect the OEM unit is something I will never understand. You can leave it as is and use a different oil galley plug for the Aftermarket unit. Leaving parts to just dangle in the engine compartment is just ghetto looking, not to mention it causes problems like this!
Im guessing the old owner didnt know that you can just unplug it and take the sensor out and the light wont come on. This way you dont have problems like this. Or just re-install it for a backup as it was from the factory and put the aftermarket gauge sender at the rear galley plug. The more things to tell you something is wrong, the better IMO.