Whilst I haven't tested one I imagine there are 2 wires you connect to the car to make it work.
The heater element is just a circuit going round which is fed 12v to make it it heat up.
So a simple continuity test to make sure the circuit is complete is all that's needed.
Put your multimeter in this setting:
![20230828_114938.jpg](./download/file.php?id=168926)
- 20230828_114938.jpg (107.52 KiB) Viewed 161 times
When it reads 1 that means there is no circuit continuity and the heater element will not work.
Touch the 2 prongs together and it should read 0:
![20230828_114952.jpg](./download/file.php?id=168925)
- 20230828_114952.jpg (89.9 KiB) Viewed 161 times
So now put a prong on one wire and the other prong on the other wire. Doesn't matter which way round. You want a reading of 0 or anything thing below 010.
Any higher than that and there is resistance on the circuit, either a break or close to a break in the element.