There are two types of grounds with a transmitter system. RF ground and electrical ground. An antenna can function without the mount being attached to ground because the RF siganl will inductively couple to the flat plane of metal (roof) beneath the antenna. Depends on antenna design. The SWR is the key to your success. If it goes up when a ground is attached then that is indicative of having a difference in ground potential between the electrical and RF ground. So if SWR's are better without the second ground attachment, go with it. This discussion goes round and round here. To qulaify myself, I have been an Extra Class amatuer operator for 16 years, hold a General Radio Operators License and a degree in EE. Take it for what it is worth and have fun.