300E Battery Discharge

[Walter1]

I have a problem where after sitting for 2 consecutive nights, the battery goes flat to approx 8 volts. I trickle charged the battery over a 10 hour period over the weekend and the same occured today after 2 days sitting.
I disconnected the negative terminal of the battery and plugged my test light again the negative terminal to ground and the light lit up. With the light engaged, I disconnected each fuse on a trial and error basis and found fuse C to dim the light but not to shut it off. This was the only fuse that had any effect on the light.

If my diagnosis is correct, could it be something in that circuit and can someone advise next steps?

Or is it the battery that should be suspect here as it is over 3 years old and an aftermarket 780 CCC.
Battery is being charged by the alternator OK as when I last checked last week, the alternator would charge at 13.8 volts when 2000 RPM with heater and lights on.