Top advice.

Just had a look on wiring diagram for mk1 Focus

Fuse 4 (50a,large fuse) in the battery junction box feeds the relay, the relay is also in the battery junction box. Pretty much inline with fuse 4. Engine run relay is connected to the HFS relay too, as Thrush says, these only work with engine running (and if the battery is charging)

Switch get power through the central junction box (fusebox behind the glovebox) and is also controlled by the central timer module (GEM module which controls the amount of time the screen is on)

Fuse no 58 7.5a (in the fusebox behind the glove box) feeds the switch side of the circuit.

Without know exact build date of car that's only a guide for 2000> Focus mk1.