More Impala wiring misery

OK, I've extracted the battery and tray, and looked all around the battery area, but somehow I can't seem to see (from overhead at least)
the damage to the wiring that the mechanic who looked at this car before reported. I looked at the work order and it also says something about "connection problem between pin 59 of connector 1 and pin 2 of the ALDC." I assume ALDC is the OBD-II connector, but what does that have to do with the no-start condition?
A little background: the car's climate control blower motor, the gauge cluster, AND the starter operate intermittently or not at all. It got so bad that the car flat out stopped starting at all, and right before that the cluster wouldn't register anything but speed. SECURITY appears when the key is turned on, but that never seemed to affect the startability of the car at all, and it never did a fuel shutoff or anything like the Passlock II was set off. The local dealer replaced the ignition cylinder and programmed it for the car, but that wasn't even the problem. The mechanic that told my fiancee it was because of wires being corroded by battery acid got the thing working just fine, except for codes for the EGR valve pintle position and the transmission TCC performance appearing on an OBD-II scan.
I have wiring diagrams but I am still at a loss. I just DON'T SEE ANY DAMAGE. Is there somewhere I'm not looking or something I might have missed? This doesn't make ANY sense.
The car is back like it was before that mechanic looked it over--no start, SECURITY, oh yeah...and the P/R/N/D/whatever fades to black after about eight seconds of turning the power on in the car, too.
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