GSM-DCS mobile phones
All phones have hands-free kits and the dealers are ready to install them but, most often than not, they are not really motivated to keep the original looks of the interior untouched.
It's less important for more modern, Bluetooth phones and kits where there has to be no direct contact, the phone can be put anywhere, even left in your pocket or purse, but with more traditional kits, the most delicate question is the phone holder. It surely is a matter of taste but, for instance, I'd hate to have the holder somewhere on the dashboard where nearly all installation workshops put it. It also depends on your phoning habits: if you use it very often and it's you who places the calls most of the time, it is important to have it in your sight to avoid any danger of accident while you are operating the phone.
However, I found it much prettier to have it on the center console behind the handbrake lever. If you have rear electric windows fitted, the operating switches are located at the end of the console on both sides. In addition, there are many openings there covered by dummy panels. You could, for instance, relocate the two rear window switches to the two corner positions, one behind the other, on the left side of the console. Then you have a clear area above the remaining three dummy panels where you can fit the phone holder. Depending on its size it might make using the small compartment behind the ground clearance lever rather difficult. So, your phone will lie one its back between the front seats and although this is a less advantageous place to operate it while driving, it is also less intrusive.
The rest of the system is easier. The microphone can go either to the driver's side A-pillar, or better yet, to the ceiling lamp—in this second case, both you and your passenger can speak directly into it. Remove the sun shield and the passenger side A-pillar trim panel (you can pull it out after sliding it carefully upwards). Fit the small microphone to the frame of the ceiling lamp on your side (the kits ususally contain a double-sided adhesive or Velcro tape to secure the microphone in place with). The wiring can disappear behind the ceiling carpet through the small circular cover of the roof antenna of the stereo and then travel together with that antenna cable down between the glovebox and the side wall of the car.
In some hands-free kits the speaker unit is separate from the main control unit. The best place I could find for the speaker is behind the upper glovebox. As you remove it (two screws at the bottom and four plugs in the side walls), you will spot the air cooling-heating rectangular tube behind the glovebox opening going its way to the side vent. The speaker unit can sit on this plastic tube, secured in place by two or three nylon ties (like the ones used to keep a wiring harness together). The two speaker grilles in the dashboard are not recommended: put a nice pair of tweeters driven by your radio there instead...
Different phones have different control units. I have a Nokia which is rather small and flat. As the upper glovebox is removed, you can see some room on the left (assuming left-hand drive) of the upper glovebox. But you could also place the unit on top of the mentioned vent tube, side by side with the speaker unit (or these two might be actually in one unit in some hands-free kits) or attach it to the car side wall in the passenger side footwell.
The only remaining item is the antenna. You can drill another hole on the roof to put your antenna there but it works just as well if you hide it somewhere—at least this is my experience. I finally found two feasible places: either inside the cabin, also behind the upper glovebox. If you buy an antenna with a magnetized base, you can simply let is snap to the side wall so that the antenna itself is located horizontally, just in front of the speaker unit. Or, you can fit it in the bulkhead cavity behind the engine, behind the washer reservoir, beneath the plastic grille the wiper arm is coming through; secured in the same manner, using its magnetized foot to snap it to the steel side wall. In the second case you can take the same route with its cable which the washer pump wiring and the windshield wiper motor use.
In theory, the antenna should be vertical and it should be not inside the cabin which acts as a Faraday cage. In practice, however, I found no difference at all (and I tried with a Nokia phone with the net monitor switched on; in this mode of operation it can display the measured field strength of the radio signals).