"Fineshot", thanks for the reply and I have a fairly simple answer as to why you did not have success with these. If I recall correctly, the Chevy Cavalier and Lumina's had their antennas mounted in the rear of the vehicle, on either side of the trunk. The issue for you was more related to signal loss due to the type of coaxial cable used in these installations. The cable is inexpensive, basically the cheapest they could use given the frequency spectrum they were trying to satisfy. Secondly, the cheap coaxial cable is very susceptible to engine noise, vehicle computer noise and so on. Coupled with the insertion of this device, which changed the electrical properties of the radio setup, I would have expected this to work in either of these vehicles very well.
On my vehicle, the antenna is fender mounted on the front-passenger side of the vehicle, a coax run of less than 5 ft, if that. In addition, the Ford vehicles with this type of antenna setup typically use RG-8x because of the close proximity to the engine, vehicle computer, etc. Perhaps this is why on my vehicle, this setup works quite well. As I noted earlier, my Chevy Trail Blazer from 2002, didn't work worth a dam. I have also noticed that GM, Dodge/Chrysler/Jeep vehicles have much noisier electrical systems, most likely because of their choice of grounding.