In my '87 US 924S it is fuse 3 on the "Additional -top- fuse panel.
It is easier to just replace the relay than test it.
To test the relay you need to remove it and heat it in a pan of water with a thermometer in the pan. The relay is connected to a meter and when the water is heated sufficiently the relay will "close".
The relay contacts arc and pit inside. May fail occasionally again. Easier to just replace with new.
Get the same temp---3 different relay temps available.
J_AZ