Why do you ned to put a harness on him?
Does he freak it if you touch him?
If he is a lap cat at all, i would recommend getting him to relax in your lap, or ona bed by petting him, and then as gently and as relaxes a possible, slip it over his head.. then if he freaks out let him run away with it on him like htat.. eventualy he will calm down, and you could prolly pick him up and manuver him with out too much problem to clasp it.
Otherwise ask your vet for some kind of sedetive for you.. tell them you neeed to take the cat for a car ride and he gets sick. The will prolly give you some acepromezene, or the like (like 2-3 pills, couple bucks) and then you have to con him into taking it. (crush it and put it into can of wet cat food WELL mixed)