HSI, RMI, VOR1 display question?

On some aircraft, with an HSI, when the GPS/NAV switch is in the GPS position, the RMI and/or VOR1 display also switches to the GPS selection and will not show reception any longer from a VOR signal within range.On some other aircraft, under exactly the same conditions, the RMI and/or VOR1 display will retain the VOR Navigation reception and display ignoring the GPS/NAV switch configuration. They respond only to a VOR freq. signal within range regardless of the GPS/NAV switch position. With this configuration one has both GPS navigation control by the AP, but can still receive and observe the VOR signal as well, which is a preferred adder IMHO.Does anyone know if, and if so where line items are in config. files that govern these two formats? In other words, where and what needs to be edited so that RMI's and/or VOR1's that are affected by the GPS switch selection will ignore this selection and continue to monitor VOR frequencies?Thanks:RTH