Uniden PC66a High SWR's

I have an older 40 channel Uniden PC66a from the early 90's or 80's ? anyway,it's been worked over sometime in the past 15+yr's.I bought it(cheap) awhile back and have been trying to put it use.I can only tell you that it talk's loud,and has a switch on the side, for 40 lower channels and a knob on the other side, for an echo board(connex)? Both add on's work,but when keying the mic(Astatic636L)it slam's the needle on the RF meter all the way over and then some. I seem to be having a very hard time getting the radio/antenna combo tuned in, it's showing very high SWR 3.0+ and have a combo SWR/Watt meter hooked to run permanently in the cab. I'm not sure about the Rf gain knob it seem's to have been juiced or ? if you turn it wide open it seem's to put out alot.The channel 9 button seem's to do something else ? not go to channel 9. I am running a single antenna- 4ft Firestick with an Astatic 9ft RG-8 grey wire, I grounded the door,run the power off of the Battery,the radio is mounted solid to the passenger seat frame with a heavy garage built mount . The truck is a 2008 7700 series Int/Navistar Quint Axle End Dump with C-13 Cat 470hp 8spd Lo-Lo . I have not tried a set of Battery Jumper cable's from the frame to the mirror mount to see if that help's ? I'm running out of idea's ??
Thank's in advance for any help

 

I am running a Braided Ground stap, to the door and truck body, have not checked to see if it may be missing a cab to chassis ground strap or a bad grounding there. I may try different grounding (jumpers) to see if that help's lower the SWR along with changing the Antenna with a known good one as this one will pull in skip and talks well but not very far ! and we still haul gravel and crushed rock/pearock in the winter everyday just about Thank's for the response