What I said is that the engine and transmission used in your Santro are both from Mitsubishi - and require their original TCM (trans control Module) to run.
If you just bolt it and dont wire it up - it will forever remain in 3rd gear and never ever shift. To make it function i.e. shift correctly you need the original wiring harness as in the manual car there is only one ECU - in the auto car there would be two units, an ECU and a TCU on top of it.
The TCU takes various signals from the ECU and is spidered into the wiring harness - SO THAT IS WHY you need a whole wiring harness. If you go look around you will find that the newer plastic case ECUs with 4 plugs wiring is not available readily but the older style, metal case ECU with 3 plugs is plenty (this setup came in the shoe shaped santro). Thats why I said its much easier.