Yes, hybrids do. And hybrids have high voltage NiMH or Lithium battery packs which can supply thousands of watts of power to run the electric a/c. And hybrids have electric motors/generators that can produce large amounts of electricity. A 123 has neither of those things.

The 6500BTU number above doesn't feel right. That is a small window air conditioner. Compared to the volume and cooling available from an automobile - even an old Benz - I don't believe it is correct. I've always heard 1-2 tons (12K-24K BTU/Hr) for automotive air conditioners.

A typical home central air compressor consumes approximately 1000 watts per ton. Let's assume the electric automotive compressor is the same efficiency, and that you want 1.5 tons of cooling for the 123. So you need 1,500 watts of power. At 13 volts, that means the a/c compressor is drawing 115 amperes of current.

Most 123 models came with a 65 ampere alternator. Are you seeing the problem here?