Chevrolet: 87 Chevy Caprice Wagon losing water, chevy caprice, caprice wagon

Question
My '87 Chevy wagon loses water (rather quickly)then overheats when I run the heater or defrost.
It was actually blowing steam into the car through the heat vent on the passengers side. I can't find the temp. gauge on the dash (there's a spot on the dash where it must be, but it's never lit up so I guess the gauge doesn't work).      
It doesn't lose water when running the AC. Can you give me some idea of what the problem might be? I don't know a thing about cars and am dirt poor. It would be nice to have some idea of what the problem is before I take it to a mechanic.

THank you!

Answer
OK
  I THINK THAT YOU HAVE A BAD HEATER CORE ! THE BEST THING AND THE EASY AND FASTEST THING IS TO HAVE A  P.S.I. TEST THIS WILL SHOW WHERE THE LEAK IS BUT WHEN YOU SAID IT IS COMMING INTO THE CAR FROM THE HEAT I BET IT IS YOUR HEATER CORE. BUT GET THE COOLING SYSTEM CHECKED PRESSURE TEST DONE THAT WILL SHOW ALL OK
GOOD LUCK AND IF YOU DO NEED A HEATER CORE AND YOU ARE GOING TO GET IT DONE AT A SHOP ASK AT LEAST 2 OR 3 PLACES FOR HOW MUCH AND DONT GET A JIP GOB GUY TO DO IT GET IT DONE RIGHT TO SAVE YOU TROUBLE IN THE FUTURE P.S. DONT TRUST A USED ONE GET NEW OR REBUILT ONLY
GOOD LUCK
ROGER