We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Stephen
saying something like:

I had not realised that the top heater is only used for boost and the
bottom heater only used for overnight. I always thought they came on
together. I realise this is due to the cable again but it does seem
silly having a top heater that is never used.

Clue - Bath and Sink.