My ac and heater works fine but the problem is when l turn on my heater its good nothing happens but if l turn on the ac, water starts to build up on the passenger side floor board, so l cant even turn the ac on or my passenger side floor board will be a big puddle in my truck. Also if its a part l need to replace what is it and where would be the best place to buy that part.

Sounds like the heater core is leaking. If you do not know what is wrong with it then you may not quite know how to replace it. l would take it to a mechanic and pay the diagnostics and let them look at it for a correct answer. Most dodge dealerships with service advisors that know anything can answer that question for you over the phone.

water drain stopped up, blow it out or run something through it. heater core has nothing to do with it.

you do not need to by anything for that one ,all to you need to do is clean the drain out for the heater box,those get stopped up over the years and cause this too happen,on the passengers side under the vehicle there is a small black rubber hose coming out of the firewall run a piece of wire in this and open it up,that will stop it from leaking on the inside like that,it takes about 30 minutes to clean one out real good,good luck.