Sorry, but I'm still not clear about your situation. An autopilot system basically has three components; 1. a control unit, 2. an ACU (processor) with a heading sensor, and 3. a drive unit (rotary, linear, etc.)
It sounds like you have an original ST6002 control head that doesn't work and alongside that a second ST6002 control head that now also doesn't work. You don't mention what ACU(s) you have or if they work but the original was likely a Raymarine ACU400. Does it work? If you have a second ACU, does it work? Then it sounds like you've got an original Raymarine Type 2 Rotary drive actuator but you didn't note if it works or not, plus an added linear drive of unspecified origin that doesn't work (maybe).
It sounds like someone added a redundant system with a linear drive. Does this have it's own ST6002 control unit AND it's own ACU (making it totally redundant)? Is the linear drive back at the quadrant? We can only guess at the configurations you have, but it sounds like you would have little or no re-engineering to simply replace the failed component(s) of either the original rotary system or the added linear system. Fix the one with the least broken parts :-) if you don't want the redundancy.