I've got this junk Game Boy Advance, seller told me it won't power on. The unit was suffering some corrotion and the...

It turned out it wasthe switch, actually. I found this video very useful as it follows all the traces on the power line until the first MOSfet. Checking the continuity step by step, I found a loose ohmic contact into the switch. An energic IPA scrubbing solved the problem and I'm satisfied like hell now! :)