Speed Sensor problem only when electrics are in use???

I think I have a problem with my 2002 Focus 1.6 speed sensor. I have only had the car two weeks, and was told by the previous owner that he had the speed sensor replaced about 2 months ago.
The ususal speed sensor problem of the speedo dropping to zero and the car becoming jumpy like its not getting fuelled properly occur.
However this problem only occurs when i have the cars lights on and then switch on the heated front and rear screens. Its like power to the speed sensor suddenly goes when I am drawing a lot of current from the battery, when I switch the heated screens off again the speedo immediately starts working again.
Could the problem be that the battery is on its way out or could it be a loose connection with the speed sensor or some other electrical fault or even alternator?
Any ideas would be appreciated before i take it to a garage to be looked at.
Thanks, Dan
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Battery, alternator and especially the engine to chassis earth first.
Tim..
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