Ahh, the comfort of sliding into a pre-warmed bed in the middle of winter can't be beat. But sooner or later you'll have to face the question: How do you wash fabric that's full of wires? Before you do anything else, look around the outside seam for a manufacturer's tag for any prohibitions against washing, drying or dry cleaning.
Remove your mattress pad from the bed and disconnect the power cord.
Load the mattress pad loosely into your washing machine, being careful not to wrap the heating coils too tightly around the agitator. Add the amount of mild laundry soap according to the soap package. Do not add extra.
Presoak the mattress pad for 10 to 15 minutes.
Adjust the setting of your washing machine to the delicate cycle and the water temperature to warm wash and cold rinse. Turn on the machine and allow it to wash as usual.
Remove the heated mattress pad from the washer and shake it out to release any tangles and wrinkles. Load it loosely into the dryer.
Set your dryer control on the delicate setting and low heat or no heat (depending on the available settings on your machine); dry for no longer than 10 minutes.
Pull the mattress pad out when the dryer finishes. The pad will still be damp; the object is to avoid shrinking, so you don't want the heated mattress pad to be fully dry when you remove it from the dryer.
Grasp the pad with one hand on the center top and the other hand on the outside top and stretch into shape. Repeat this procedure in the middle of the pad and then the bottom; continue with the other side until your heated mattress pad has been fully reshaped.
Drape the mattress pad over a clothesline to let it air dry the rest of the way. If you don't have a clothesline, you can hang the heated mattress pad over two chairs.