
Hi June,
in line with other replies you could a) put the harness on indoors and then have a short game with him etc for 5 minutes to distract the immediate association of "nasty harness" & nasty car". Then, b) sit in the seat beside him in the car until he's more settled.
On a personal level, I don't like harnesses and would only use a cage - but that is personal preference and whatever is most practical for your vehicle. If you can have a cage however, I'd scrap the harness now :)
If using the harness method, I'd recommed getting him used to sitting in the car, harness on but not restrained and with the engine off. Do this for a few minutes at a time, say 3 x daily gradually increasing period over say 3/4 days and then fix harness, sit with him for reassurance and continue as before for 2 days, car still parked. Then try having engine running, work with him a minimum of 3 x daily for a couple of days and if he's more at ease try moving the car slowly for say only 50 or 100 yards. NEVER reverse in the early stages - this confuses them and can cause travel sickness even just coming out of the driveway ;)
Beyond that it is a case of slow and gradual build up to avoid negative experiences in the car such as feeling sick. When he can be driven without getting himself in a state for say 5 minutes, stop the car and take him for a walk (preferably in an area like parkland but that depends on your location :) ) Spend longer with him out of the car than the journey back will be.
At 14 weeks he's very young and it should be relatively easy to overcome his anxieties providing you don't pressure him. If you desensitise him by having him in it, initially unrestrained and car parked and then increase periods at his pace then with patience and habitual opportunities to learn cars mean either fun or even just settle down for a nap then he will learn to relax.
Good luck, Teri :)
ps. if the harness is a real issue for him, why not try putting it on him during the day without going near a car (constantly supervised of course) :) Just have him wear it to play a game with you and even to eat his meals?