[COBALT]

First, is it AC or non-AC? There are two types. AC heater cores have a funky in/out, with an acute angled connection. Non-AC is your real easily identifiable rectangle with 5/8" in and 3/4" out (bottom and top). You can get them at NAPA for about $35.

Second (if non-AC) the inlet to the core runs from the intake to the bottom of the core (5/8"), and return coolant from the core comes from the top and runs to the water pump (3/4"). Heatwise hot fluid comes from the intake, and is dumped out by the heater core before returning to the pump cooler than it went in. If you have it mounted backwards then your coolant flow is the same (from 5/8" to 3/4"), but the coolant is going with gravity rather than against it. That shouldn't make the core "take a dump", but it might effect heat transfer. You want coolant in the core as long as possible to get as much heat out as possible.

Third, changing it can be a pain depending on the condition of your heater box. If you have non-AC like I do I ended up unbolting the hood hinge at the bottom, and sticking a wood block under it on top of the wheel well to get enough clearance to get it out (you have to take the heater box out).

If your heater box is in bad shape it might be worth it to get a weather stripping and sealant kit from LMC. They include all the foam pieces plus the weather stripping to seal up the box to keep the hot air either inside the box, or transferred to the cab (and not to your engine compartment).

Strip the box down and clean it. The heater core is held in place by two metal rods that are bent to hold it. There's a couple of sheet metal screws to take out. Oh, and also: lube the mechanism that shuts the heat door, and make sure you get that heater control loop back on the little peg when you put the heater box back in!

Have fun.