Aircraft Spruce offers harnesses for avionics well below my cost.  Research their avionics options for details.  I can build harnesses for used avionics on a case by case basis providing the owner sends the factory backplate with associated connector and backshell.  There is no reason to buy used avionics without the tray and connector assembly.

Buyer beware Garmin doesn't sell their threaded backshell retainer separately.  This is due to customers buying installation kits for competitor slide in replacements of Garmin units.  If you buy used Garmin avionics, make sure the seller sends all associated connectors and mounting hardware.