overheating,

I have a 2001 Jetta VR6 and I have just replaced the crack pipe, thermostat, thermostat housing, all seals and gaskets. Problem is that now the car overheats and the bottom return radiator hose stays cold so I figured that the thermostat wasn't opening so I replaced it with another new one ( 3rd one now ) but the problem still is there. I also put all 3 presumed bad thermostats in hot water and they all open. What could be causing this ? The fans don't come on unless I turn on the AC. Could it be the ECT? I found out that when I turn on the heater, it blows out nothing but cold air. Another funny thing is that as soon as I turn on the HEATER the fans come on as if I turned on the A/C. Thanks in advance for any help of info. I really would like to get my wifes car back to her . I have been working on it for 4 days now. Please help.

well i have 98 mk3 jetta 2.0 and i have been having problems with the heating and i replaced the thermostat and water pump and sometimes it will runn perfectly fine and not go past the middle heat mark but sometimes it does and my fans will not come on unless my A/C is turned on but on random occasions at idling they will so i have determined a electrical problem idk if that helps but check your water pump and also radiator fan relay switch make sure ur radiator is equal warmth top and bottom