Flat Battery for the first time

[Rolebama]

I went to start the car Monday night and discovered a flat battery. (It is down to 6.5 volts.) So yesterday I took it off to charge and fitted the other battery I held back when I got rid of the 306DT. It was ten days since I had last used car.I did an amp check before connecting - terminal and found only a 70ma drain. Anyway, went out last night to use it, and noticed the light for the cigarette lighter was on. I thought I would have a look at it today. Car started OK, and when I turned the lights on, the cigarette lighter went out. I do have a slight damp problem with the car, but I have a big bag of dehumidifier grains in the car which is currently only slightly damp. The cigarette lighter light has so far stayed off, so as with most intermittent faults, it's probably going to be a buggar to find. Any suggestions in the meantime would be appreciated. Thank you.

[MixMan]

This sounds like 'Wet footwell syndrome' to me. Have the carpet up in the drivers footwell and have a look see if you have a swimingpool in there.
Carpet may feel dry on top, but it's notorious under there. Aso under there, is the main connections between the front and rear looms, and the connectors corrode like mad and the electrics go mental.
If you take the connectors apart, be Really careful as the pins might all start falling out
I had this and ended up having the wires cut and soldered together to stop future cases of it happening again.