Heated leather seats will not heat up. Switch lights up and all fuses are okay. Is there a hiddedn relay I am not finding? Is there a quick way to determine if the problem is in the heating element under the leather seat cover?

Question edited for clarity.
Question moved from Spanish Car company 'Seat', pronounced See-At.

Check Fuse 13 (Heated Seats)

https://fuse-box.info/chevrolet/chevrolet-uplander-2005-2008-fuses-and-relay

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1. Blown Fuse: A blown fuse can prevent power from reaching the heated seat system. Check the owner's manual for the location of the fuse for the heated seats and replace it if necessary.
2. Bad Relay: A bad relay may also prevent power from reaching the heated seat system. Check the relay for the heated seats and replace it if necessary.
3. Bad Heating Element: The heating element in the seat may have failed. If this is the case, you will need to have the seat cushion removed to replace the heating element.
4. Loose Connections: Check for loose connections in the wiring to the heated seat system. Make sure that all connections are tight and secure.
5. Control Switch Failure: The switch that turns on the heated seat may have failed. If the switch is not functioning properly, it will need to be replaced.
6. Bad Wiring: The wiring in the heated seat system may have become damaged or frayed over time. If this is the case, you will need to have the damaged wiring repaired or replaced.

Heated Seats
This system allows the electrical heating of the front seats on passenger demand.
The components of the system are:

• the seat heating elements located in the seat cushion and backrest.
• the heated seat control switch, located on the seat side trim panel.
• temperature sensors located in the seat cushion heating elements.
• heated seat relay.

Do both front seat heaters not work ? Do you know how to test electrical circuits with voltmeter ?
PINPOINT TEST J: THE HEATED SEAT IS INOPERATIVE Test Step Result / Action to Take J1 CHECK HEATED SEAT SWITCH OPERATION

• Key in ON position.
• Press the heated seat switch on the driver and passenger seats.
• Are both heated seat switch indicators illuminated?

Yes
If both indicators illuminate, Go To Pinpoint Test K . If only one indicator illuminates, GO to J5 .

No
GO to J2 . J2 CHECK THE HEATED SEAT RELAY CIRCUITS

• Key in OFF position.
• Disconnect: Heated Seat Relay C3250 (located under the driver seat).
• Key in ON position.
• Measure the voltage at heated seat relay C3250-86, circuit 1003 (GY/YE), harness side and ground; and between C3250-30, circuit 566 (DG), harness and ground.
  
• Are the voltages greater than 10 volts?

Yes
GO to J3 .

No
REPAIR inoperative 1003 (GY/YE) or 566 (DG) circuit. TEST the system for normal operation. J3 CHECK HEATED SEAT RELAY GROUND CIRCUIT

• Key in OFF position.
• Measure the resistance between heated seat relay C3250-85, circuit 57 (BK), harness side and ground.
  
• Is the resistance less than 5 ohms?

Yes
GO to J4 .

No
REPAIR circuit 57 (BK). TEST the system for normal operation. J4 CHECK THE HEATED SEAT RELAY

• Install a fused jumper lead between heated seat relay 3250-30, circuit 566 (DG), harness side and heated seat relay C3250-87, circuit 1064 (YE/LB), harness side.
  
• Do both heated seat indicators illuminate when pressed?

Yes
INSTALL a new heated seat relay. TEST the system for normal operation.

No
REPAIR circuit 1064 (YE/LB) to switches. TEST the system for normal operation. J5 CHECK INOPERATIVE HEATED SEAT SWITCH INPUT

• Key in OFF position.
• Disconnect: Heated Seat Switch C320 (Driver) or C321 (Passenger) for the inoperative heated seat.
• Key in ON position.
• Measure the voltage between heated seat switch C320-2 (driver) or C321-2 (passenger), circuit 1064 (YE/LB), harness side and ground .
  
• Is the voltage greater than 10 volts?

Yes
GO to J6 .

No
REPAIR circuit 1064 (YE/LB). TEST system for normal operation. J6 CHECK CIRCUIT 57 (BK) FOR AN OPEN

• Measure the resistance between heated seat switch C320-4 (driver) or C321-4 (passenger), circuit 57 (BK), harness side and ground.
  
• Is the resistance less than 5 ohms?

Yes
INSTALL a new heated seat switch. TEST the system for normal operation.

No
REPAIR circuit 57 (BK). TEST the system for normal operation It's not that simple , being a fuse . It's probably the relay or the heating element .
PINPOINT TEST K: SEAT DOES NOT HEAT BUT THE HEATED SEAT INDICATOR ILLUMINATES WHEN PRESSED ? Does it ?
Your best bet would be to take to a qualified repair shop .
Do

You need to check if 12 volts is getting to the connector at the seat first then we can tell more

It is a fuse. Go into your owner manual to locate the fuse or just look in the fuse panel as they are labeled.

check under the seat, there are wires and connectors that can get loose from moving the seat back and forth

you will need to check the fuses first and if they are okay. then take a test light and check the switch for the power seat itself and see if you are getting power to and from the switch. if all is okay check power at connector for heated seat and if all is okay the heat is bad or it maybe the switch i have seen both. good luck

Swap it with a relay you know to be good and see if it works again,pick a relay that is not important to the function of your car.

You took some seats with heat from a truck and put them into your truck that never had heated seats?
Well im betting that you need to do some research about the truck that has heated seats and see what relay (s) you need in your truck.and additional fuses you may need. AND any additional electrical connections that may need to be done. Plus you would also need a switch to turn on the heated seats as well. And if its automatic then you would need some sort of temperature sensor-switch. I hope this helps you out in getting started on this project. it shouldnt be too bad to do..and good luck with the seat upgrade!

Ok it is obvious since the light is coming on you are getting power to the switch. It could be a heated seat relay. Im fairly sure jeeps use a relay after the switch that is where I would start. Check and see if the Heated seat relay, and fuse are good