Naturally your mechanic doesn't know what is wrong - he caused it!
Odd problems do not develop all be themselves mysteriously after
some repair is done. If a new problem occurs right after a repair to
something unrelated, odds are almost 100% something done during that
repair caused it.
It sounds like the "mechanic" disconnected or broke some wiring under
the dash. It sounds like you should take it somewhere else and somehow
try to get some money back from the original repair.
He damn well DOES know what's wrong, he just doesn't want to tear the
whole dash apart to find the connector that he pulled out while replacing
the heater core. Not that I can blame him; it's no fun working in there.
"C'est un Nagra. C'est suisse, et tres, tres precis."
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