I have a mini 0806 - cost around £75 IIRC - with a polarising filter fitted, and it has been more or less faultless in the year it has been installed. Not wanting wires and so on, I got it fitted by a (MB dealer) recommended guy, and it just works!
Only problem that I can level at it is that the internal (non removable) battery does not hold a charge so well now, so a series of shorter trips can sound the low battery warning when the car powers down - it is wired into switchable ignition source, so fires up with the car and shuts down after a preset delay on turning car off.