Heater not getting hot may be a sign of low in coolant/ air getting into or trapped in the cooling system. If the problem has only occurred immediately after refilling the coolant system air in the system may be most likely cause if the car over heated prior to replacing the radiator and the engine still runs hot there is a possibility of head gasket problems. The most accurate way to test for a blown head gasket on a gasoline engine is to test for the presence of carbon monoxide in the cooling system. Snap-On, Matco (part # CO 2000), and Napa sell a tool to check to carry out this test. Another link to a supplier of this tool is http://www.etoolcart.com/combustion-leak-detector-lis75500.aspx. Carbon Monoxide is only present if there is a blown head gasket or cracked cylinder head. http://www.arrowheadradiator.com/head_gasket_or_combustion_leak_test.htm I think the napa part number 700-1006. http://www.blockchek.com/instructions.htm . Signs of a blown head gasket are coolant loss (frequently have to top up the coolant level of the engine without seeing an obvious leak), experiencing over heating or rough engine running or the presence of white sweet smelling steam from the exhaust. A badly blown head gasket may allow engine coolant and oil to mix.
You might have a crack in your degas bottle (coolant reservoir) I had the same problem. I also didn't notice it leaking on the ground. After reading a lot of posts, I discovered that this seems to be a pretty common problem. The leaky bottle allows air into the system and slowly leaks your coolant out. I wanted to see if this was the case in my 2000 LS. I pulled the old reservoir out and sure enough - there was a crack in it. I bought a new one from Amazon.com for about $65 (part number RB603207) and put the new one in. Then I bled the system as specified here: http://townhall-talk.edmunds.com/direct/view/.f1168da/29 Good luck!