You likely don't need anything like a drill-powered brush for just regular dirt/pollen. I've always just used car wash soap and a wash mitt. Then once a year I'll get a Raggtopp cleaner/protectant kit and use that.
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https://www.amazon.com/Raggtopp-Convertible-Top-Care-Kit/dp/B0002KKIVO
I bought Raggtopp from Amazon. Then I bought the 'horse hair' brush along with it. Worked for me. Read the instructions though, the vinyl on the car needs to be completely wet before applying. I don't remember paying that much for it but that was 2 years ago.
https://www.amazon.com/Raggtopp-Convertible-Top-Care-Kit/dp/B0002KKIVO
Personally, I went with RaggTopp because it is recommended by the manufacturer of the premium/twill top fabric. 303 is also very good and I would have went with them if I had the vinyl top.
Edit: Just make sure you get the right 303 for the job. Fabric for the premium/twill tops. Vinyl for all other tops.
Is your convertible top cloth? If so, this is what you want: http://www.amazon.com/RAGGTOPP-Convertible-Top-Care-Kit/dp/B0002KKIVO
This is what I use on my Miata's cloth top.
For vinyl tops, there are special cleaners and you can follow up with 303 Aerospace.