Grounding. I have a 96 Freightliner and a Magnum S9 radio and everyone says when I transmit it

Question

I have a 96 Freightliner and a Magnum S9 radio and everyone says when I transmit it sounds like I am talking out of a hole except when I run in rain or in the morning when the dew is on then they say I sound great and they can hear me for a long way away. Would this be a ground issue and where would the issue be? Do i need to ground the radio or the antanas better and how do you suggest? Thankyou for your help.