I have an old 1990 Toyota 4X4 Pickup and have plenty of ground plane but last week I bought a Wilson 4' No Ground Plane Fiberglass Antenna Kit (it's the one with the slide SWR adjustment up by the radio connection). Usually these antennas are for fiberglass/non metallic vehicles or those lacking sufficient ground plane. Anyway I mounted the aluminum bracket back about 3" from the left taillight, I have no bumper or tailgate so I had a perfect flat surface to mount the bracket. The only metal to metal are the screws going through the bracket. I ran the 17' NGP cable that I wrapped in plastic conduit all the way up the frame and into the cab. The plastic conduit ends right at the slide SWR adjustment. I set the SWR a couple times and finally got about 2.0 on channel 40 and 1.5 on channel 1. Channel 19 is right at 1.0.
I can pickup transmissions from Canada to South America and beyond. I live in Jacksonville Florida and the local signals darn near blow me out of the cab!
Of course I'm only transmitting the legal 4 watts but I just wondered if anyone else had this good a signal using NGP systems on metal vehicles?
It picks up better than my 30' Solarcox on a 50' tower! I'm amazed.

All i can say is: something must be wrong with your solarcon base antenna.

__________________
"Y'all mind hanging back? You're jamming my frequencies" -- Tangina in Poltergeist

Yea, the base coax is dry rotted and full of cracks and holes.
It was only for comparitive purposes. If I rewired my tower I'd pickup and transmit darn near anywhere.
I'm just shocked at how well the mobile NGP antenna picks up being on a vehicle with plenty of metal ground plane. I was always told an NGP system would not pickup well close to metal. Half the antenna is 3" away from metal surface.

It seems like the antenna you're comparing to isn't exactly in the best shape in the world. I would be surprised if a mobile (or any) antenna didn't at least match it's performance.
The 'No Groundplane' type antennas are sort of misleading. They may not use a vehicles metal body for the 'other half' of the antenna, but you can bet that they do use something for that 'other half'. In most cases, that other half is the feed line. And since your vehicle does have a metal body, I would assume it's being used too. Maybe in a 'round-a-bout' fashion, but still being used. If it suits your needs, then why not?
- 'Doc

(Put a new feed line on that base antenna anyway.)

The new antenna instructions say the ground is through the cable running back to the radio. I have even added plastic conduit around the entire rubber coated "tin" cable and an aluminum non-metallic mount for the antenna. The only metal ground is at the CB itself through the negative battery ground. The antenna I replaced was a 6' whip grounded at the body and frame. Even at near perfect SWR the whip was no comparison to this Wilson 4' NGP.

and the negative post on the battery is probably grounded to the vehicle's frame, making the whole vehicle part of the ground plane.

If it's working good, don't change it

so it recieves good but how does it talk?