Don't you have the standard Santorin insallation of a Raymarine Type 2 Rotary drive (not a linear drive) with a chain turning the wheel? And is that controlled by the ST6002?
It's not clear from your posting what the problem is - has the rotary drive failed or has the autopilot controller failed. If it's the rotary drive and that was the original on your boat, Raymarine no longer supports that and it's likely it's more than just brushes. You'll be far better served by getting a new one (about $1,500). You can either control it with your old ST6002 or with a modern unit like the EVO or a B&G NAC3, etc.
Good luck with it,
Craig SN68 Sangaris