Probably the radio's illumination circuit doesn't draw enough current for a 25 ohm resistance to give enough of a voltage drop to dim the display sufficiently.
If you get a chance to play with it some more, try adding a 50-ohm or a 100-ohm resistor in series with the rheostat--RadioShack #s 271-133 and 271-135 respectively. These come in 2 packs, so if one isn't enough, add another.
