In the case of A-star's tacho what your SA said is true., It doesn't comes with the wires. the mgp wiring harness costs about 3-6k(i don't actually remember the price, anyhow it is more than 3k). But they gave me an option to get it fixed with an aftermarket wire to reduce the expenses. So i gone with it.
I dont know about the connection, since they were not ready to tell me about it.
Ya you are right, since its not the regular routine of their work, they literally hesitate to do these kinds of work.
Finally i got it fixed after about 8 months of waiting and by frequently communicating with them. Finally MASS communicated with the maruti and they provided them the directions for the connection.
Tacho on board is really fun to drive