Passenger seat heater not working in my 2003 volkswagen jetta

There should be a connector under each seat which connects power to the heating element. It is likely either clipped to the seat or tucked under the carpeting. You'll almost definitely need to remove the seat to get easier access to it, though it should be possible to find/access it without removing it, just more difficult. Once you find it, disconnect it. You can use any multimeter with a continuity tester to test the circuit. First, check the DC voltage coming from the wires that come up from the floorboard. If you are getting voltage there, you will want to check continuity of the element itself by probing the two wires with the meter set to it's continuity testing selection. If there are more than two wires you will need to determine which is the common wire and which are the "live" wires. With the power disconnected you can test the element to figure this out easily. For example if your heated seat has three wires, test all three for continuity in pairs. If one wire always shows continuity no matter which other wires on the same plug you touch that should be common.

If the element tests good (in other words you find that all the circuits show continuity) and you are getting power from the plug that comes from the vehicle itself, you might need to check the resistance of the element. It is rare to have a bad element that still shows continuity, but it is possible that the resistance may have for some reason become lowered and the element therefore does not heat up.

Try these and let me know how it goes!