Randomly failing iPhone 3G doesn't charge

hi,

A friend gave me his iPhone 3g, because there was a sim card stuck in it. I removed the screen, took the sim out and put it back together, but I realized a screen connector wasn't properly connected. So i opened it again, and connected the connector 1 cable, but then i found out that the phone was turned on, and i couldn't turn it off because the screen wasn't connected. The screen would't work, even when properly connected so I waited till the battery drained, put the phone back together and the low battery charging screen appeared so the screen works, but the earpiece speaker didn't, so i opened it again, connected everything properly, but then it would't charge or turn on anymore!

I didn't change anything at the logic board or charging circuits. I cleaned the dock connector and tested the battery with a multimeter and that worked fine, but I don't know if the dock connector works properly. How can I test it?

I think the dock connector is the problem but i'm not sure.

Also notable: it has been damaged by water (home button broke, power button broke doesn't always work) but that was two years ago. Could this have caused the random failures?

In short: I need to know how to test if the dock connector still works, or how to fix a non charging iPhone 3g

Thanks in advance.