I've seen some strange behaviour with dimming as well.

The headlight sense line in my 2000 VW Passat is 0V when the light switch is off, then ranges from 2-12 volts, depending on the dash dimmer setting.

When I first installed the unit in my car, I thought I did something wrong because it was very dim all the time, regardless of whether my lights were on. I thought I must have accidently hooked it to the +12V or something.

Then I discovered the dimmer setting in the empeg and cranked it to 100% (it's not all that bright anyway)

Even at 100% it still dims a bit when I turn the lights on, which is fine. However, the strangeness is that it gets confused sometimes about what state it should be in.

Sometimes it won't dim, and sometimes it won't un-dim with the lights. I've actually seen it get brighter when I turned the lights on!

So, although there may be h/w at issue for you, I suspect there is a s/w bug too. It's probably a boolean flag that gets tripped when the voltage changes, but the circuit has a capacitor on it that is intended to keep it from getting tripped on small power spikes or drops.

Also, what state the rest of the player is in might play a part. For example, when a song is paused, I don't think it will switch modes. Not sure, but I think that's what I remember.

I can't speak to the 80% bug, though. I'll have to try it.

-Joe