Heater control relay location

Electronic climate control power module . Don't know what relay you replaced because this vehicle has no blower motor relay . 1990 Cadillac DeVille Power Module

next to blower motor

It gets power from right hand maxi fuse block , Ignition 1 fuse, 40 amp. through ignition 1 relay.
1998 Cadillac DeVille ' DeVille, Eldorado (VIN E/K) Service Manual ' Body and Accessories ' Wiring Systems ' Component Locator ' Document ID: 519143
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Engine Compartment, Near the Left Strut Tower

the heater core in a 1998 Cadillac is very tricky to replace.
start with getting a pressure test to determine where the exact location of the leak is, it may not be the heater core.
if you do decide to pull the dash apart to change the core make sure the battery is disconnected for at least an hour to allow the airbag system to discharge.

Hello: Check the fuses in the fuse box under the hood, drivers side also check the fuses in the trunk fuse panel on the drivers side. If the tail light fuse is open that will keep the instrument lights out also.

I just replaced a blower motor on my 98 cadallic deville and it is located on the fire wall not easy to remove because of clearence

The fuel pump relay is in the fuse panel under the hood on driver's side on top of wheel well. The fuel relay should be gray in color. The correct location is listed on the fuse panel cover

If for the heater it is a blower module on top of the evaporator core housing. If for the cooling fans it is inside the front fender behind the headlights. That is a module also tice the size of a normal fist

Are you sure that you don't mean the cooling fan control module? It should be located up front next to the cooling fan. It is square shaped and has a long square shaped plug going into it. Reads the temperature of the coolant and passes the information to the engine control computer (ECC). The ECC receives analog information and signals the cooling fan switch to cycle the cooling fan on or off, as required.

this problem is caused by a defective air temp control blend door motor, this door and actuator motor is located inside the dash between the heater core and the AC evaporator core, it shifts airflow through the heater core or the AC core or mixes or blends the two for the desired temp at the outlets, the motor will have to be replaced. This is a very common issue.