Depending on the version of RES you're running, there should be a way to add a CONTRAST setting to the main menu. Again, my version of RES (5.5.9) is almost certainly not the same as yours, but this may still assist:
Click on the MENUS button in the config section--it's near the top on my version
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Then enable the Contrast setting in the pulldown menu.
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Write it to the radio, and then you should now be able to set your contrast from the main menu.
Hope that helps--perhaps someone with the exact radio and RES version may have something more specific.