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Presuming your truck is between '87 and 96, take out the glove box, then you'll be looking at a plastic cover. Take out the screws pull the cover off, and the heater core will be right in front of you. Take the hoses off the outside, and pull the heater core out - you'll have to turn it and wiggle a little because it's a tight squeeze to get in and out. Put the new one in, reconnect the hoses, cover back on, glove box back in, done in an hour or less.