Assuming the Quest has a 2.5mm mono jack like my SPIII then you need a part 1567 (was part 174) , price £22, 2.5mm mono to 3.5 mm stereo isolated lead . If you only EVER run your quest off battery power & never bike power, then you could make to with part 1561 (was 171) , price £4.
or if you want to have a mobile /ipod connected as well as the quest
Part 1282 twin isolated input adaptor
part 1293 3.5mono to 3.5 stereo
My Ram mount came with a 3.5 female connector
.... either Garmins, or an aftermarket bracket, that comes complete with power supply and amplifier.
The Quest doesn't have its own amp, like the bigger generation Garmins, so all the power and sound amplification is done in the bracket.
This will have a 3.5mm mono socket to allow connection of the headset either via the Autocom or straight into your helmet if you prefer.
DON'T be tempted to try and power the Quest direct as it runs at 5 volts not 12v as with other units!