ROM and power LED

[Doobrey]

Nope, the code that changes the power LED is inside the ROM.

Also, just because the screen hasn't gone red doesn't mean the ROM is OK. It's the same as the LED, the code that puts up the  coloured error screens is in ROM too, so if the ROM is dead or  there's data corruption before the checksum routine then it can't put up the error or flash the LED

[Doobrey]

What happened, major battery leak or something ?
Have you tried doing a continuity test between the address and datalines of the ROM and CPU sockets?
BTW, you might want to inspect the Gary chip.. it's responsible for decoding the ROM address space and generating its chip enable signal.
Also, one of the CIAs has a pin that enables/disables the rom overlay (ie tells Gary to decode ROM at 0x0000000 to 0x00800000 at bootup)

Edit..
 Found it, it's the CIA at U300. You could try swapping the CIAs around, if you're lucky and it boots it'll save you a lot of soldering  :-)