CAT3 ILS roll out guidance is checked every 6 months in JAA countries by flying at 50 feet in a special role BE20 - see
www.flightprecsion.co.uk
A vehicle driving in front of a CAT3 ILS WILL affect the localiser signal and if close to the Glide Slope array (approx to one side of PAPIS) then that will be affected - no problem for a seasoned manual flyer but some autopilots may trip out.
CAT3 has extremely good reflection surface ahead of the runway threshold to ensure a very smooth glideslope signal.
Note that over the last 200 feet or so on the deccent the G/S signal is also affected by snow on the ground / water (eg: rwy closest to sea at Barcelona, Leuchars etc) (ie: on that reflection surface).