Anyone done this? Is there a grommet behind the battery?

What do you need to power? there are a few permanent live fuses in the glovebox which can be tapped into.

Usb sockets, dash cam, radio and metre for a taxi.

Have a read through this should help.

 

http://www.briskoda.net/forums/topic/393595-my-dash-cam-fitting/page-2?hl=%2Bdash+%2Bcam#entry4788763

Two-way radio systems generally need (and should use) a separate fused feed directly off the battery as the OP is asking. The electrics will go through fron the fuse box under the bonnet and it may be possible to get a power feed through there that has enough capacity to run the meter and two-way radio. The Dashcam and USB device can be run from piggyback fusing off the main fuse-board in the glovebox.