You MUST change the input going into the back of the receiver in some manner. The cable from the rooftop antenna must be removed and replaced by the cable coming from the ground connection. As mentioned above, a good quality, high isolation A/B switch can handle this job as well. Additionally, be sure you are not trying to use the outside Cable Input connection as that is not meant to be used for satellite. That connection "might" be able to be modified for that purpose but it will not work as factory wired.
What I post is my 2 cents and nothing more. Please don't read anything into my post that's not there. If you disagree, that's OK.
Can't we all just get along?