So, will this set up be grounded correctly? Is it the radio that needs to be grounded or the antenna?
What does the bushing on the spare tire carrier look like is it insulated? if so just run a ground strap to the frame of the vehicle ground are very important in order to get range. Irving lets setup a day and i will look at your set up and give you pointers.
You sir, are a specialist in the art of discovering a welcoming outcome of a particular situation....not a mechanic.