i don't know the 147 specifically, and i'm yet to tackle my GT (which is probably the same), but have down this in my 156.
check at the base of the windscreen, on the lefthand side of the car. so, near the bonnet hinge on the left, remove any rubber wind/debris 'blocks'. also remove the black plastic trim/shroud that covers the bottom of the windscreen where the wiper blades are.
there is probably a grommet in the area below the windscreen. you could either pierce the grommet to pass the wire or remove it and cover the sharp metal edge of the hole with some split conduit tubing or similar.
from inside the cabin, remove the door rubber and any dash trim in that area (not sure if your car is LHD or RHD). you can probably feed the wire in from the bonnet and grab it from the inside. may need to facilitate this using a cut piece of coathanger wire or similar, firmly taped to the end of the powerwire.
in the 156, i fed 0awg powerwire (~15mm outer diameter) without major issues. will need to do the same in the GT.