Pilot reported loss of control while conducting an ILS approach to a non towered airport. The reporter believes the event was caused by a vehicle driving by the ILS critical area.

Narrative

The incident occurred on final approach to Runway X at ZZZ utilizing the ILS approach to localizer minimums. Approximately 1 mile after passing the FAF (ZZZZZ)the autopilot began uncommanded turns and climbs and descents. We got as low as 1600 feet; 700 feet AGL; with numerous left and right turns. By the time I got the autopilot turned off we had turned left approximately 90 degrees. At this point I told ATC that we were going missed to due equipment issues and that I was just trying to maintain heading and climb. With the autopilot off I was able to regain control of the aircraft. After determining that my instrumentation was operating correctly I turned the autopilot on in only heading mode. This functioned normally so I engaged altitude hold upon reaching 5000 feet. This also worked fine. These observations led me to believe that the ILS ground equipment was the issue. I then requested the RNAV X at ZZZ; received clearance to the IAF; and shot the approach to landing with no issues. Prior to my flight that day I saw a NOTAM that the ILS would be out for a few hours. I asked the airport manager if the maintenance tech could have it up for our arrival. The tech said this would be fine. I assumed that this must have been the cause for my erratic flight on final approach. After landing I spoke with a couple of friends that were watching from the ramp to see my approach and landing. Apparently an unauthorized vehicle had driven down the taxiway and through or very near the ILS critical area at approximately the same time as the incident occurred. They knew the timing coincided as they could hear my position calls on CTAF. This unauthorized vehicle had been informed of proper procedures for accessing the airport grounds. They had previously performed work on the airport. More must be done to stress the importance of adhering to these procedures and the consequences for those who don't. Perhaps changing gate codes more often.