Here's my radar detector/GPS hardwire implementation. My goal was to have neither device attached to the windshield with suction cups.
For the radar detector, I used those velcro-like (but different) adhesive strips that are provided when you get an Easy-Pass for driving on the thruway. They are securing the front end of the radar detector to the dash hood. I then stuck a rubber adhesive pad on the back end of the radar detector to hold it level, since the dash's hood slopes downward as it approaches the windshield.
For the GPS, I stuck the round adhesive disk onto the dash hood.
To dress up the wiring as it makes its way toward the windshield, I used some slitted plastic wiring tubing. I then dressed the wires along the windshield. To keep myself from accidentally pulling the wiring out of position when I'm connecting/disconnecting the devices, I pressed a 1-inch long piece of clear tubing between the dash and the windshield right at the point where the wiring disappears between the dash and the windshield. This tubing holds the right end of the wiring securely in place.
When I got to the speaker grill, I removed the grill and dressed the wiring through the speaker cavity because the GPS wire is thick and I couldn't fully press it down along where the grill meets the A-pillar (not enough flex). The underside of the grill has plastic ridges that partition the grill into "compartments". I strategically trimmed away some of this partition material so that the cabling wouldn't keep the grill from laying flat and flush with the dash. Now the grill is holding the left end of the wiring securely in place.
I used an add-a-line to add a 10A circuit to the "cigar" fuse position. The radar detector runs off 12Vdc power, so I connected that wire directly to the add-a-line. The TomTom GPS requires a 5Vdc converter (which is contained within the plug that comes with the GPS), so for that I wired a cigarette lighter socket to the add-a-line (down in the cavity where the fuses are), then inserted the GPS plug directly into that socket.
I soldered and used heat-shrink tubing for all of my wiring interconnections.
No more device wires coming down in front of the dash/head unit/ventilation controls area!