'81 Eldorado - diagnostic read-out

Hi folks, I've got a problem, and I'm looking for some advice. I purchased an '81 Eldorado in July, '02. On the highway, the speed control wouldn't hold, so I
opted for my hi-way car and let the Eldorado sit. In fact, it sat all winter. I'm talking a Saskatchewan winter where the temp dips down to -30C and lower, and holds for weeks on end. The battery froze, and I fear it affected the computers. It simply won't run anymore. I've since replaced the ECM, to no avail, so it's been in my mechanics shop since - trying to figure out the problem. They tell me that before I can do a diagnostic read-out, I have to replace the control head that plugs into the back of the heater/AC control module. When you push the right buttons on the face of this module, you're supposed to get a numerically coded diagnostic read-out on the face of the module (in the dashboard). THAT"S what I'm looking for. There's also 2 factors that come into play.
Apparently, the '81's have either a 'design 1' or 'design 2' configuration. Mechanic advises that we're probably looking for 'design 2'. Further, the rear window defroster is on a separate switch, so the plug-in at the back that I'm looking for can not be configured for a rear window defroster.
Can anyone out there point me in the right direction? I'm new to this 'newsgroup' thing, so I'm certainly open to suggestions for posting my query to other newsgroups
Any help would be muchly appreciated.

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