How to open battery door to recplace battery?

Could be the switch going bad or a short in a wire. If it had ever been in a accident that is where i would look for a broken or smashed wire.

Unlock the rear door and climb in that way or call a locksmith to open the door for you

Don't do what Fred said. this is a complicated switch-mode amplifier - you'll need to pass it to someone who knows these.
after you recplace the obviously burned parts, you need to run it up in test mode and go through the test procedures until you find the other faults that caused it to fail.

http://cgi.ebay.co.uk/Replacement-Remote-Control-LG-42PQ3000-Plasma-TV-/110616756578?pt=UK_AudioTVElectronics_Video_RemoteControls&hash=item19c145c162

I finally got mine open a few minutes ago by prying it with a knife. A thin screwdriver would have been safer.

Sounds like an intermittent connection on the door switch.. Remove the switch, clean the contacts and recplace.

I dont believe those parts are available by themselves from sony.

The remote does have a battery. It either has a small door that slides open or you remove a screw to separate the two halves of the case. The battery is inside.

If you saw smoke, Im sorry to tell you, your going to need to take the amp to a repir personm or shop. Good luck and please let us know how you made out with this repair.