If you still have the Grom kit, I would suggest you to install it up front.
When I install them in the trunk (OEM satnav carsonly) I always remove the back seat in order to fit the BT dongle in the B or C pillar. Works well.
Too far out when left over in the trunk alright.

Or if you do not have it anymore, I have the new model here that I just received yesterday, BT3, same device but with BT module included in it, so it is a bit cheaper too. 160 euros supplied. Mic included. Streams music too.