What is it

That is the "Water Flow Control Valve"" It controls the flow of water into or NOT into the heater core. If it's leaking, you might as well replace it now while the seats are out. If you plan to remove the dash, wait and do it when the dash is out. Replace the hoses (all 3) at the same time to save yourself rooting around there again later! Just my $.02

Carl

I agree with BambiKiller240. It is very easy to replace, albeit a bit messy with the possiblity of radiator fluid leaking into the footwell. You might want to drain your radiator first or at least open it to relieve any pressure. Remove the carpet and padding so as not to leak on them, although chances are, if your asking, they are already wet with fluid. I'm not sure of the official part name but I recall it being called a "valve ****" when I replaced mine. It is, or at least was, an inexpensive part.