So after 5 months my backup camera isn't working properly, it doesn't come on if I put the truck in reverse. It used to work properly.amd come on when I went into reverse

Settings battery

Rear camera on

Wired from tail light to the if I remember correctly the purple wire on head unit

Plugged into brown input on the radio"not the dongle wire"

It will come on if I turn on camera view, but it won't automatically come on if I put the car into reverse anymore.

I've reset the radio and that hasn't worked

Im having the same issue that you had.

On the store wich i installed my NEX5000 and my camera, they said that my multimedia video INput and aux, are not working properly, are broken. 

The rest of multimedia functions are working normally, so im thinking that could have something wrong with this history... 

back to you case, i didn't understand the part that you say that:  

" Wired from tail light to the if I remember correctly the purple wire on head unit

Plugged into brown input on the radio"not the dongle wire"

Could you explain more for me please? 

You wire from the reverse light to the wire on the head unit, that way the head unit gets triggered to switch the display to your rear camera

actual position

http://www.manualslib.com/manual/787646/Pioneer-Avic-8000nex.html?page=11#manual

Page 11  explains the wire

Check the connection for the sensor wire. Since it previously worked, you must have installed it correctly. Maybe it came loose. Do you still have back-up lights when in reverse gear? If not, might be a fuse blown with resulting lack of reverse-signal to the sensor wire.