Yaris increases the RPM whenever (aftermarket) cruise is engaged no matter what is the road condition. Other Factory Fitted cruise control cars dont do it on same road.
If the ground wire is connected with a wire which have further components on the way to ground then those components will be in series with the cruise stalk circuitry causing the voltage to drop at all the loads this may cause a malfunction as appearing in case of Yaris.
The connection is a hit and trial way, if any one gor a success he noted which wire to splice for ground and then this info circulated as word of mouth and now same is practiced by all installers. None is trying to get the issue resolved as it works for the time being.
When you explain the above to the installer they do not understand it and say cruise ki light tu aarahi hai na, gari khud race le rahi hai na, tu phir kya masla hai. Sab theek hai.