807 Sliding door problem

[cornish807]

My rear sliding door stopped working electrically a while ago and was very stiff to operate manually. I have just replaced the left sliding door motor with one I bought from ebay. Fitted it, adjusted the tension and the door moves freely, manually. However, it still won't work electrically. I removed it again and plugged it into the right hand side and the motor works fine. So I haven't bought a dud motor.
Does anybody know of any reason why it still doesn't work? Do I need to reset the door control ECU? Any help would be much appreciated.