I am happy to report that my car does not seem to generate much QRM at all on HF and what little there was (on 40metres) the noise blanker on the 706 deals with.
I am not so pleased with the boot mount I bought for the HF antennas. It looks like the one below
Mine is unbranded but I have seen these branded as Sirio mounts. The connectors to the centre pin and earth under the mount are not in any way weatherproofed. Worse than that tightening the nut (as I needed to do) around the SO239 connector can cause the coax to disconnect and break away from the mount. This happened to me and I had to remove the mount from the car and solder the coax back on. Yes the mount does work and it holds the HF whip on fine. It is however poorly designed and its a pig to fit! Not clear in the picture is the plastic dome on which the SO239 sits which is mounted on a smaller metal dome. The plastic dome is free to slide over the metal one below altering the angle of the antenna. All very well but there doesn't seem to be a way of stopping it moving. So the antenna will not stay at the exact angle you want.
I guess its a case of you get what you pay for as these mounts are fairly cheap. I am looking for something better now as I don't think mine will last to long!