I know this thread is a tad old but saves me starting a new one, when there's no point!
Was this problem resolved, I only ask as I have just installed my 'Midland 48 plus' cb in my disco II, constant live (fused 2 amp) running direct from the battery via a cb noise supressor) I have it connected to a mag mount (I was advised to stay away from gutter mounts by Gwyn Lewis 4x4 specialist).
The CB is transmitting as I can calibrate the SWR meter, but when i switch to "REF" to get the reading (still transmitting) the needle does not budge, I have tried going through the channels but it makes no difference.
I did do two things to my setup, I installed the plug myself and soldered the connections inside the plug (first I thought this was dodgy but I did a continuity test and it all seems fine so there doesn't appear to be any shorts in the connections)
Secondly I had to remove the old wiring from the mag mount and I soldered the new wire directly to the mag mount antenna pin inside (I have the 'Sirio Mag 145 3/8' - four screws and you can access the internals), I did find that I had to cut the wiring off as I could not un-solder the wire, This is where I'm guessing things have gone wrong, but I can't see the reason why as I actually soldered it to the mag mount antenna pin and a continuity check confirms that there is a connection).
I spent best part of the day scratching my head thinking that it might be so perfectly setup it's not actualy reading anything on the SWR but somehow I find that hard to believe.
I tried adding an earth wire to the chassis, and moving the mag mount to a few different locations on the roof, I have tried three different patch leads (I have confirmed that all the leads are fine, by doing a continuity test on them), and two different microphones (just trying anything to rule things out, I also have two SWR meters and both read the same), mic gain is at max, it is configured for the UK.
I have been reading a load of posts on this forum and got a few things to check, but I'm none the wiser to actually fixing the darn thing. :frusty:
Any help/advice would be gratefully appreciated,
cheers.
Update:
I have tested the cb on different country settings, I don't get any reading on FM however I do get a reading on AM of 1.3 ish swr on certain channels, guessing this is a good sign but I find it strange that there is no reading on FM.
Anyone think my setup is faulty or perfectly fine?
Thanks.
EDIT:
Checked with a few drivers at work, it's transmitting perfectly, I have since moved the mag mount a few inches back and the swr needle is moving slightly so it would seem the setup is spot on, which is a first! 
