There is indeed an option to turn off the Eco function (this light) in the 1.5 version. Please check the manual if theres a mention of the same for 1.3 gli.
I wanted to install the cruise control as well. Heard its just plug n play. Went to two dealerships. The ones who were honest, said if anything happens to the car and Indus finds out they will not furnish the claim for warranty. The other dealership said, sir please have this cruise control installed along with the trumpet horn. No warranty would be void.
Unless you have alot of motorway travel then i would advise u against it.