Again as said above - check all earth connections and the main battery terminals at the battery end and the starter end. They can cause all sorts of lights coming on and off.

Alternator might not be charging the battery due to a dirty/loose connection.

My advice (before rushing out and buying a new alternator or battery...) is to pull off each main connection from battery/body/starter/alternator rough em up with a bit of sandpaper so that they are shiny again (rather than a dull grey colour) apply some vaseline sparingly to both the crimp and where its bolting to - to ensure a good connection and help against future corrosion. Vasoline is a pretty good electrical conductor and keeps moisture out.

The small earth connection in the boot came loose on my orion and cause all sorts of weird stuff. If I put the indicators on, the fog light switch would flash and at the rear end the sidelights would flash intermittantly along with the indicator bulbs. Pressing the brake also made the fog light switch illuminate.