High level brake light

I have found the wire to the high level brake light in my 1996 x300. It seems a simple install to add the brake light. ( purchased new old stock jag light) A YouTube video shows a very simple install. But I am getting no power in the cable feeding the light. It seems you plug a relay into the boot fuse box. The boot fuse box does not have the wiring base for that relay.

Anybody know where the relay has been located. ??

The paperwork in the car states the relay should be in the boot fuse box to feed the high level light. Fuses checked and all systems working that feed the rear lights.