Defrost causing misfire?

When i run my defroster my engine gets a misfire. This doesn't happen normally or with the ac on. What might cause this? Thank you

I have a gmc yukon 2002 5.3 2 wd

The Devil2019-08-13T01:45:59Z

You could have a failing charging system. Check the alternator, the battery and all the connections. Worn out sparkplugs will misfire. To narrow this down to a tighter problem site, pull the OBDii trouble codes.