I spent a Saturday cleaning my wheel wells but it's not futile since I don't drive the 500 on rainy days. I'm also prepping for a concours so I have an excuse!
The best methods I've found for removing wax from tight exterior trim areas is actually a wooden toothpick. Wood won't scratch the paint and can easily remove wax from lettering in the lights, your rear badge and any trim. Q-Tips soaked in a quick detailer are excellent when cleaning rubber trim around the rear windows, sunroof and doors. You should follow up with a rubber/vinyl conditioner which can also be easily applied with Q-Tips.
If you do decide to use a brush you should stick with Boars Hair detail brushes, even a soft toothbrush is too hard and will create small scratches.
Nylon brushes are great for engine detailing but try to be gentle on painted surfaces as they will scratch. Again a boars hair or horse hair brush will work as well and won't scratch.