something keeps pulling battery down

I have a 2005 chevy tahoe and something keeps pulling battery down i un-hooked negative side of battery and hooked a test light as i was told to tr , to neg battery side and to neg cable and light lit up i went through all fuses pulling one at a time to get light off to located problem well after pulling all fuses light never went off and now ac works when it wants to goes off and comes on and heater wont now blows but no heat.

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