I did this last year on the very same car, a 150 Veloce same year, it's a simple plug and play fit. The light to say its on or off is in the middle of the speedo, as it's been said you should find a white multi plus when you take the lower cowling off the steering column.
The 147/GT I've no doubt fits but it's a different size and look to the correct one for a 156 and would look right on the car, there are enough correct ones out there, try and buy one that's provided with the lower cowling just to save you the hassle of cutting the hole required in yours, although it is only a 5 min job!!
Attaches with one screw IIRC, and no need to do anything other than plug it in and away you go
75 in 2002, 156 in 2014, GT since 2015....