My 2008 dodge magnum will not start.

Hey,  so I have a problem.  My magnum will
not start.  About a month ago, I left my key
in the car, not running but in the ignition
with the radio running over night. The next
day, it was completely dead. I bought a
brand new battery after jumping it and it
dieing again. After installing the battery and
driving to the gas station, my speedometer
would jump around and various dash lights
would cone on like my brake light,  engine,
tire pressure. Pretty much every light that
could come on did. After a few minutes of
driving, it would act normal. Fast forward a
few weeks, my car died again. This time I
sent it to a mechanic who checked my
alternator and battery, everythING checked
out fine. Saying he could not find anything
that would cause the intermittent problems.
Last night, my car died again. So I had it
jumped and the whole drive home all my
dash lights were on, I couldn't turn on my
headlights without using my high beams,
my speedometer was stuck and it sounded
like I was driving in the wrong gear. Sorry
for the long message, but u wanted to paint
the entire picture.