Question about heater?

[88GT 5pt0]

My heater only works when I put it on the highest fan setting and the defroster doesn't at all. What might be wrong with it and how do I go about fixing it? Theres also a lot of cold air comming out under the dash on the passenger side when its on. Also it seems to be really draining the power because whenever I put it on the headlights and the dash lights dim. I have a system but it doesnt cause the lights to dim, even with the system off the heater makes the lights dim. I plan on putting in an alternator from a 94 soon