What color lights are you getting on you AC charger jack? Please download and run Coconut Battery so we can get a better idea of what's going on. There's a new version but I don't know if it runs on the older machines: http://www.coconut-flavour.com/coconutbattery_2.6/index.html
We are probably looking at either the DC-IN board or the battery connector. Visually examine them for corrosion. Please let us know your results. Thanks