My RDS had the same problem. If it's the same thing internally, the little spacer that keeps the knob in place has broken/shattered and now the knob is basically loose so the switch has no tension and the connection is really questionable at best. I can't think of a better way to explain it. The upside is that if this is your problem, the issue is physical (not electrical) so you can fix it with a sheet of something hard and a Dremel. The downside as you probably can tell is getting into the thing to fix it without ruining your whole RDS (like I did- tore the wiring out :S)
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