I've tuned several of those over the years. If I ever decided I needed something shorter than a 102" whip the Sirio 5000 line of mobile antennas is at the top of my list as far as the CB side of things is concerned. That being said I, personally, would use something other than a magnet mount. They can cause problems that a more permanent solution wouldn't have, and make other potential problems worse. Operative word, can, not necessarily will.
When it comes to grounding the radio causing a difference, several things could be happening, and the magnet mount may be related, but not necessarily.
Radios don't need to be grounded to function, at least as far as transmitting and receiving are concerned. It is more a safety issue. Your radio is actually grounded already, the negative power lead is also a ground, and while many people run it straight to the battery, many others run as short a length as possible to a chassis ground. Their are advantages and disadvantages either way you run the negative lead.
Adding a different ground wire, as you are doing, should not "mess up the radio", and in fact should have no effect at all. Very likely you have either a common mode currents issue of a ground loop issue.
Questions:
- How is this ground wire "messing up" the radio?
- How is the radio bracket grounded, it is bolted directly to bonded metal or is a wire running from the bracket to the vehicle's chassis? If a wire is used how long is it?
- If you add a three foot jumper and a barrel connector between the SWR meter and the antenna, does SWR change noticeably? If so how much?
- Are you running an amplifier, and if so how big is it and how is it wired to ground/negative?
The DB