Windshield Washer Reservoir 1991 560SEL 80k Miles

I have an odd situation in that I was going to add a little water to the Windshield Washer Reservoir on my 1991 560 SEL and the outlet tubes were very hot the one closer to the fire wall was scorching hot. The water was very hot in the tank. Weird, is there a heater for the reservoir. I disconnected one of the three connectors and the washer works fine I don't know yet if it affect the heat problem. Anyone know the answer. Thanks Barry