Idle sits perfect if there's a high current draw
 

Hi, I've just registered but have been learning from the forum for some time. I've bought a 2004 continental GT that has a slightly rough idle. The interior and number plate liggts also flicker in time with the idle. The car has been smoke tested and no vacuum leaks, there are no misfire codes. What I've found is if I switch on the rear window demist or both heated seats, the car Idles perfectly. Can anyone help?