Reverse isn't engaging. on 1994 Mercedes-Benz SL600

A couple of weeks ago it would take a little playing with to get it to go in to reverse. Now it just won't at all. The shifter works fine, just no go. All forward gears are fine. Someone told me they think it's a solenoid that needs to come out and be clean and that will fix it. The has just under 100,000 miles on it.

Asked by for the 1994 Mercedes-Benz SL600