SWR too high.

I have a Cobra 29 and am trying to get it to work in my company truck. It is an 08 Volvo with the antenna studs in the mirrors and a power supply built into the dash. I have tried the built in studs and got an antenna light when I keyed the mic to calibrate the SWR. When I inherited the truck it was missing the the stud on the driver's mirror and have to figure out how the back comes off the mirror so I can be sure the ground is good and the stud washer is in place. I've also tried external antenna mounts and a premade twin antenna wire. I have ground wires going from the external mounts to the ground straps on each door. I can calibrate the SWR with that set-up but when I check the SWR on the individual channels, the lowest I get is a 3 and most of them are pegging the meter. I should have mentioned, the premade antenna wires are 18ft long and that is to each antenna. The antennas are top loaded firesticks. I have been using he internal SWR meter on the radio. I am confident the ground on the external set-up is good. I have tried placing the extra wire in a couple of different places. Do I need to shorten the antenna wires?

I'd appreciate any help I can get. The company doesn't service our cb set-ups any more so I'm on my own.

Chris