Fitting - Black is ground 0V- ground?

Red - 12v+?

Blue/Purple -  the signal wire?

Grey wire unused 

what do I do with the yellow wire ?

You should have signal in to the converter, and signal out of the converter to the speedo 

Thanks Ian so where is the signal out?

Can't remember the colours, but your converter intercepts the signal, it goes through the converter, i.e. in and out, so the signal coming to your speedo from the speed sensor on the gearbox goes into the the converter, then out and to your speedo altered. It intercepts and alters it before the speedo itself. Red/black are positive and negative. Mine (when I bought her) would drop off at higher speeds and the speedo would give up, as the positive was wired straight to the cluster and it didn't have enough juice (common problem), so I think I took the switched positive from the radio with more juice to the converter, worked great since. Hope that helps somewhat.