[99 Neon R/T]

Leaking head gasket,freeze out plugs any one of a dozen things could be the problem or it could be a combination.Chocolate brown antifreeze sounds like alot of stop leak.And too much of it could explain the heater not working that stuff will clog heatercores prettt badly.Take it to a good quality shop and have drain and flush the coolant system and get a pressure check done.That should tell them where the leak is.You may get luckt and it be a 2 dollar freeze plug or it may be a 1200+ head gasket job.