In my checks of every connector in the engine bay to find the cause of increasingly frequent no start situations I found a relay like module spliced into the heat protected cable that connects to the gearbox (reverse switch?) and wiring loom just next to the downpipe. Any idea what it is? The car did seem to run without the cable connected but why on earth would someone want to put a module in that connects to a socket on the gearbox? The only connections to the module are ins and outs from the cable it is spliced into - colours white, brown and green.

No electronics mods to engine and I bought the car (an STI V) new so the thing could only be alarm/tracker/reverse or foglight related as far as I know. It was rather crudely taped into the loom so it's not likely to be factory.

Could it be for speedo altering from kmh to mph?

Could be - does the speed sensor live in the gearbox?