1998 GMC Jimmy Serious Heating Issue?

[spottedtrotter]

Ok so about 8 months ago before I actually got this car the battery was unhooked for an unknown period of time. And ever since the car has had issues with heating and cooling. If it's cold outside both my AC/Heat blows cold. And when it's hot outside both blow hot air. The thermostat was replaced maybe 3 months ago and that didn't fix the problem. My mechanic told me it would cost 300 dollars just for them to look into the issue. Does anyone have any answers that can help me out before I fork over that kind of cash. Thanks!

I was wondering if maybe the battery being out for a prolonged period of time could throw off my climate control.