I bet the battery on the expansion RAM has leaked...
Underneath the Amiga there is a trapdoor, pop it open and you will see in there a card. This is the 512k RAM expansion. Pull the card out and see if there is any corrosion near the battery.
Actually unscrew the entire case to get at the 512k card, you will need to pull out the cards wire and switch as well (that switch you said was modded onto the case).
