I'm trying to install a Cell phone booster, but it needs to be mounted as high as possible (they suggested roof), but the Fiero is plastic. How did you guys do it?

I understand the same problem exists for CB and Ham radios as well.

Threads like these don't help: http://www.k0bg.com/ground.html

I would say the highest metal point would be the rear vent covers or the decklid vent itself (84 only)

I don't have much to add other than to say that only the skin for the roof is plastic. Take your headliner out and you'll find plenty of metal. I honestly don't really know what you're working with exactly but it would be simple to add an internal ground to the roof. An external one wouldn't be too difficult either but you would likely have to modify your roof skin to do so and I'm not sure how permanent you are wanting your install to be.

[This message has been edited by GraterFang (edited 02-07-2012).]

What you are looking to do is to provide an RF ground. There is a difference between an RF ground and an electrical ground.
In an electrical ground the current flows through the interior of the conductor. The conductor doesn't have to be very large unless you are looking at a lot of power or very high electrical frequencies.
On the RF ground the RF flows around the skin of the conductor. So the larger the conductor or the more strands there are in the conductor the better. Also, for an efficient RF ground it is recommended that the grounding conductor be 1 inch wide for each foot of distance between the antenna and the grounding point. Of course that is getting rather wide for longer distances but that is the recommendation. Just install the widest that you can.
Now some other things that need to be thought of. You can transmit off the ground strap if you happen to cut it to certain lengths. So that needs to be of concern also.
Example: 4.5 feet of ground strap is a very good antenna for the 6 meter band.
Plastic/fiberglass/SMC bodied vehicles do have a problem with transmitting antennas. Special attention has to be included in the layout/positioning of the antenna to provide the correct transmission pattern and a place for an efficient RF ground.
Some antenna companies in the past made antennas that had an internal grounding system. They weren't very good but you didn't have to try and find a grounding spot for it. I'm not sure if anyone makes them now. That's what Google is for.
Now here is something that I found on an ex-police motorcycle I bought years ago. They used a sticky backed foil spread over a large area of the underside of the top portion of the storage compartment that is on the rear of the motorcycle. Then the antenna was installed through the top, penetrating the foil. That foil was now acting as the RF ground.

So how should I make RF ground for a Fiero?

btw, the frequency would be GSM, so 800mhz, 1900mhz.

quote Originally posted by Austrian Import: So how should I make RF ground for a Fiero? btw, the frequency would be GSM, so 800mhz, 1900mhz.

That depends on where you are going to mount the antenna. And whether or not you are going to punch a hole in the body. (personally I wouldn't)
I haven't contemplated adding any antennas to my Fiero so I haven't looked into places to mount.
If I were to mount one I would look at the through the glass type for the Fiero. Then I would probably mount it top center on the windshield. Run any inside cabling over the headliner, down the A pillar to the transmitter.
Newer cars with the metallized reflective coating on the windshield pose problems for these types of mounts. It interfers with the rf coupling between the outside and inside mounts.
Another option is to mount it to the edge of the front fender somehow (trunk lip type mount?) and run as short and wide an rf ground as I could to metal inside such as the inner fender.
If you are contemplating punching a hole then I would do like I found on that ex-police motorcycle. Put up a bunch of stickie backed foil around the area I would put the antenna on the inside of the vehicle. Example using a roof mount: remove headliner, attach foil tape over a large area overlapping the foil edges, drill hole, mount antenna, run coax down A pillar, reinstall headliner.