How do you replace the heater core in a 1999 Toyota Tacoma?

First you would have to remove the heater core and here are the steps.

I removed the heater core in a 4runner without removing the AC. Some claim it is easy to remove the heater core unit without removing the dash by removing the AC evaporator and the blower motor. Well.. my experience is, it is impossible to remove the heater control unit without removing the dash. It is not hard. Best thing is you can do it if you have time and patience.

The tools I used are 10, 12 and 14 mm and Philips screwdriver.
You can see the step by step pictures towards the end of this thread.

Before doing anything, remove the ground terminal from your battery and wait about a minute.

Remember, the dash will come off with a very light pull. if it is not coming off, you might have a nut/bot some where.

You then can remove the cross metal bar holding the steering column.

Disconnect the heater/cooler selector cable and air flow direction cable. Make a note (sharpie) where the metal clip holds the outer housing of the cable.

Remove all the 10 mm nuts/bolts holding the Heater/AC control unit, AC evaporator and blower. This will give some extra space to giggle around the pull the heater core unit out.

This is also good time to check any leaks in the Heater control valve in the engine bay. IF you get hot water in the heater core when you are running the AC, it will kill the cold air as about 20% cold air is directed through the heater core.