I bought a 99 tahoe four door 5.7 350. No heat. Previous owner bypassed heater core. Need to see how heater hoses attach.

Yes you bypassed the heater core which is the radiator that supplies the heat for the passenger compartment

There are two reasons that could be the cause. The most common is that the blend door in the heater control box is non-operational and is not routing the air from the blower through the heater core. The other possibility is that the heater core is clogged and not getting coolant flow. Actually, a third (more remote) possibility is that if the car is used, a previous owner may have bypassed the heater core if it was leaking. You would have to check the heater hoses under the hood to make sure they are being routed through the firewall and not connect to each other, bypassing the heater core.

Assuming the fan blows OK but you have no heat. Go to the firewall and notice where the heater hoses enter and exit the cabin. You'll notice a metal manifold and also a rubber bypass hose that connects the two. This bypass hose is a design problem and causes the coolant to bypass the heater core which is located inside the cabin under the dash. Over time, the heater core gets blocked by gunk and you'll get no heat. All the coolant flows through the bypass hose when this happens. The fix is to flush the heater core itself. It's difficult to access at the firewall, so I disconnect the two hoses at the engine itself and attach a garden hose to flush it. Before doing this you must clamp (vice grips) the bypass hose so that all of the water goes through the heater core.

You should be able to look down in between the motor and the firewall and see if the heater hoses are attached to the heater core. Most of the time when they are bypassed it is because the heater core has started to leak. Instead of replacing the heater core (not a fun or easy job) they just bypass it so they won't leak coolant. So, if it is bypassed your probably looking at replacing the heater core in order to get heat.

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Could be that the heater core is disconnected, there are two hoses that attach to the heather core that come out of the engine, this provides hot water to the core thus making the heater work, when the heater core starts leaking water the quick and fast fix would be to bypass the heater core and connect the two hoses together, check under the hood on the left passenger side there should be two hoses about 1" thick see if there are connected together or of they seem to be connected to 2 outlets that come out of the inside of the vehicle. people do that specially if selling vehicle.

bypass it with rubber hose

Yes!

Hello bijtj.
There are no bypass hoses on the heater core. Flow of coolant out of the engine and into the core and back to the engine is flowing at all times. The heat inside your car is adjusted by means of a door inside the heater box.
If your intent is to bypass the core because it is leaking you would need to pull the hoses off at the firewall and install a joiner.

If you are not the original owner, the previous owner may have bypassed the heater core. The heater core is like a small radiator that has hot water running through it from your motor. It is inside of your dash board, near your a/c and heater controls. That is how yur car gets hot air. The air runs through the heater core making it hot. Bypassing it is a cheap and quick fix for a leaking heater core. When they get a leak, they leak all inside the car down to the floor board. Check under the hood along the fire wall. If it is bypassed, you will see two hoses spliced together. One will be bigger that the other slightly. If this is your problem, you are more thatn likely need to replace your heater core

its not a fuse if the blower motor works , you have a clogged or bad thermastat, bad water pump or your radiator heater core is clogged