How can I fix my Daf (LF45 150)lorry's indicators?

I got a 2001 ('51) plate Daf LF45 150 lorry recently, drove it back with everything working then a couple of weeks later got a tracker device fitted. After that I noticed the side and rear indicators and hazards were not working (the fronts are fine). We've checked all the fuses and relays - all are ok. My initial suspicion was that on fitting the tracker something must have happened, I have had the guy that fitted it back out and he retraced his steps - finding nothing amiss. Any ideas? (The lorry's previous owner had as far as we could see looked after the lorry very well although hadn't used it very much at all hence reason for selling it. It does need a fair bit of WD40 on things like locking fuel cap, etc which are very stiff through lack of use.)