Heya...I had the same thing happen to me. The heater core in your car has blown. There's no "easy fix" for this...it will have to be replaced. Depending on your truck, this is either a $300 fix or a $800 fix. If there is an access panel in your truck, then the fix is only about $300 (that's what it was for me) but if there's no access panel, then the shop will have to remove your dashboard, etc, to get to the core and the extra labor will push the cost of the job to probably about $800 (depending on what they charge you for labor). Call around to a few garages (or your mechanic if you have one already) and tell them the make and year of your truck. They should be able to tell you if it will be the "cheap" fix or the "expensive" one.