Replace heater fan carbon brushes?

Hi, the fan on my merc sprinter 1997 308D has stopped working, it was intermittent now packed in all together. Web research points towards the carbon brushes. I can see the fan on the bulkhead in the engine bay but just need a bit of guidance to explain where, what and how to replace them. I am firly good at most mechanical jobs but this is one I have not ever tried before. I just want to do it quickly.

Regards.

SC