M.F.
Keep wrestling, and make sure you keep winning! ;-)
However, some tips that might give you the upper hand:
Definitely start loading the car at least 10 or 20 min before you have to go anywhere.
First and foremost, STAY CALM. I know it's hard to do with a kicking toddler, but he needs to know that you can handle this without breaking a sweat. Make up a silly song about going bye-bye or getting buckled in and sing it while you get him in the car (yes, keep singing cheerily as you're pinning him down with all your strength to get the buckle fastened. When you're done with the fight, say "Okay! We're ready to go!"
You might also ask your son if he'd like to help buckle himself in. When you're getting in the car just ask "Would you like to help mommy with your buckles, or not help?" Then see if he'll help get his arms through the straps, and maybe buckle the chest clip. It may take a while to get totally buckled while he struggles with the clip, so be sure you're not in a hurry.
Another idea: Time out. You'll have to get in the car about a half hour before you need to, but when he throws a fit, tell him "uh oh! Guess you need some room time" and go put him in his room or crib and shut the door for a minute or so. When the minute is up say, "ok! Let's try getting in the car again." Keep doing it until he stops throwing a fit. My 19 mo old only needs 1 or 2 rounds of "room time" before he decides to cooperate, but he's not much of a fighter in general. My older child is a different story. It could take 8 or 10 times before she comes around!
Another idea: This will take a little planning, but call the local police department and see if you can stop by and have a uniformed officer help you out. Go ahead with the wrestling match, and when you finally get him buckled say, "Whew! That was a lot of hard work. It was sooo hard that I might need some help keeping you safe. I think I will see if a nice police officer can help." Drive to the police station and have the officer come out (a male officer may be more formidable, but anyone in a uniform should be enough to stun him into silence). Open the door to your car and stand next to your buckled-in little darling while you explain to the officer what a hard time you're having keeping him safe, and maybe you need some help, so would he please show you how to do it. Then tell the boy the nice police man is going to help you both learn to be safe. Unbuckle him and hand him to the cop, then let him buckle him. Over-dramatize your interest in what's going on for you son's sake: "OHHh, the LEFT arm goes first....ok, I can do that. Son, do you think you can remember that? Oh look! you buckle the bottom buckle first. Good job officer! etc., etc. Once he's re-buckled ask "is there anything else we need to know?" then thank him and go on your merry way. Say to your son, "I'm so glad we did that! I like it when you're safe in your seat!"