(I can't edit, so I'll add here) It seems likely that either the coolant level sensor is bad, or the connector is bad, or the connection is bad, or the wire got damaged while you were working on it. You should be able to test the sensor with a VOM, and the wiring, but sorry, I haven't had to do this so I don't know the details (like sensor open = low and closed = ok, or visa versa).
2004.5 Silverado 3500 SRW 4x4, D/A (LLY), Ext. Cab, Long Bed
engineer837 aux rad, TxC CAI, LBZ MP, Scangauge II, Nicktane aux filter head, KD lift pump, 2008 PS pump, 2011 hydroboost, Frederico Swaybars, Rancho 9000XL, Torklift StableLoads, BFG AT/KO2, Lance 835 Camper often in the bed.