I re-did a 2002 a while ago… It had a heater which I eventually ended up ripping out and tossing. I found it pretty useless without the doors and generally in southern california. I was also installing 96v lithium packs and didn’t think it would handle the extra voltage.
I had a couple of questions about it and ended up emailing this guy tommym@heatercraft.com who was pretty helpful.
Found some pics of the unit -
If it were me… I’d ditch it and go with a Mr heater or something like grant did here The Best Heater for your GEM