Yes, of course you must sue Sears and not the installer if you paid Sears and your contract is with them.
Go to your secretary of state's website and find Sears. It may be a DBA for something else, but ultimately every corporation must state their agent for service of process. For big companies their agent is usually some forwarding company like US Corporation Company, or some such name. Sometimes they have multiple similar names like Sears, Inc. and Sears Stores, Inc. , but usually they list the same agent for service. And the agent has an office staffed 9-5, usually just a hole in the wall of some Downtown office building in the capital, where a young man or woman will accept papers for their clients. Be sure to get a receipt. (Of course you cannot serve the papers yourself if you are the plaintiff).
If you are not in the capital then look up process servers in that city, call them to determine the fee for serving US Corporation Company or whoever it is, and send the papers off with a check. They will send you proof of service.
Good luck.