dancad

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

D I R T Y I 3 0

Might be the alternator man

tosheto

iTrader: (5)

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.

njmaxseltd

iTrader: (19)

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

DennisMik

iTrader: (2)

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.

Climaxx

iTrader: (4)

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.