Car wiring

[Numplumb]

Hi does anybody know what the large metal box thing with a metal grill over it with two wire connector one red and one black wire and where abouts it is fitted on Mk1 MX5 1600, bought all my parts already dismantled so I don't have any idea where things were in the engine bay and I can't seem to find any reference to it in any books etc, I'm sorting out the wiring loom and want to put the wiring down the right side of the engine for this item, also I have problem with the relays for the lights, I have a Headlight relay but the wiring colour codes don't seem to match any diagrams I can find and I don't seem to have the TNS relay.
Can anyone help please.
Regards Mick.