My 99 Maxima has 12.6 volts at the battery yet no lights, dash lights or anything else has any power.

Things started to go wrong when I would turn the low beam light switch on. All power would dump when I would do that. Charged the battery and with the light switch off, car starts and runs fine.

With engine off, when I turn the headlight switch on to low beam the power dumps again

Changed the battery and now I have absolutely nothing. The Security light does link as normal and that's it

I have 6.5 volts at the power feed to the ignition switch

Any ideas

 

Might be the alternator man

 

Start the car and measure the battery voltage. If it is around 14.5V then your alternator is good. If it is much lower then you need a new one because this one is toast. Check your fuses. Maybe the wires of the headlights are messed up and kill your battery.

 

There are additional power lines that connect to the battery + terminal.
Remove them, clean and inspect the ends for corrosion.

 

Definitely check the main 120 amp fusible link that is in the middle of the fuse box next to the battery. The alternator won't charge and a lot of things won't work, but if the horn blows, then the fusible link is OK.

 

THIS.

I'm willing to put money that this is your issue. The alternator isn't even a factor since OP said it does this even with the car off.