0.1 Amps is a little high, generally it should be closer to 0.05 amps (50mA)

If 0.1 Amps is draining a battery a considerable amount over night, I reckon the battery is knackered! if lets say its a 50Ah (50 Amp-Hour) battery then technically it "should" hold about 12V for 500 Hours at 0.1A. (But of course in the real world it doesnt quite work out like that.)

Might i also add, I bought a brand new Halfords battery for my Alfa Romeo 156, and low and behold the Radio drained the battery within 3 days of non use (bloody alfa electrics!!) but that destroyed the battery.

no matter what after that i would never hold charge as well as it did before that.

Basically my point is that the previous owner may have left something on and toodled off to spain for a week to come back to a dodgey battery which maybe ok with daily use, but is dead after that.

but thats just my two pence. Do you have access to a battery tester?

I powered the aerial booster from the powered aerial connection on the HU so it would only gave power when the radio was actually on.

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Could well be my battery is knackered. I wonder if one out of my two donor porsche 924s will fit....