Is it going in a Geneseis??
If so its a very simple job .......... one of LGBs best arranged for digitising locos, simply remove the small roof hatch, below which is the PCB on which you will see a rectangular blanking plug approx 30 X5mm remove this & plug in the MTS3 decoder plug on the end of its cable.
It is also worth noting there is an exhaust port in the roof section ( black plastic) some locos had a small black plastic plug in these t o stop rain entering the loco!! ( all My Genesis locos did) some locos were not so fitted, so it is worth blocking up this hole, just in case!!
l