hiya bill, i've asked this question too, i had a grom installed a few years ago and it was fantastic, back then they didnt have a bluetooth option, it seems to be relatively new, which is prob why all existing grom users here dont know about it. I really want to try it but would love some feedback first, if you decide to take the plunge first please leave me your feedback.
As for cabling, my first grom, all cables ran behind stereo and under the gear gator, under the middle console and into the back compartment (by your elbow), no cables visable and i loved it. This time i have built a retractable iphone holder fitted under the heated seats plate, so when i install grom, cable will come from under there so still pretty much invisible and the mic i was gonna mount on the sunglasses holder tray (where the satnav is on the better models)