Anyone know what this is?

[shaunroche]

Might it be some sort of dimmer rheostat type of thing?

It's in the boot above the fuel tank....funny place to locte it if it is!

I had an earlier sprint than this one, Aug 73 versus Nov 73, and that didn't have one...

[maitretjak]

It is a dimmer relay indeed, meant to dim rear brake and indicator lights when tail lights are on. Can be adjusted by taking off yellow cover and turning the screw (where the bulge is in the cover). It was fitted to all early cars in the "1850/stromberg" (early '73) period.
Can make your brake lights near invisible in daylight, indistinguishable from the tail lights you switched on....

[Mad Mart]

Indeed, it's a night dimming relay. It should be attached on the other side of that bracket.

[Dolly-Dimple]

How interesting, ill need to look and see if my 1850 has this!

[shaunroche]

Like I say my first Sprint was a few months older than this one and that didn't have one....was this original equipment from a certain date or an option or something?

[Carledo]

They seem to come and go without much logic attached! My 78 Sprint doesn't have one, which has saved me the bother of ripping it out and binning it!

Steve

[Mahesh]

If there were one in my car, I would bypass it, electrical dimmers are an unnecessary resistance in the circuit, and bound to corrode.

I'd want my brake lights as bright as legally possible.