Heal fast,.... a thumb injury is much like a sore toe... where ever you walk you tend to stub that toe.... since we use our thumbs often, a sore thumb becomes a real handicap....About 70 years ago, when I was in 4th grade, in a base ball game I was at short stop and caught a line drive, with the wrong hand and sprained my right thumb... I had trouble with that thumb for a number of years afterwards, well up into my teens and maybe afterwards... I didn't have the benefit of a splint back in those days, so I was a little slow healing.... hopefully you will heal much better and quicker than I did.
Also glad that no other injuries from the car crash... rear end crashes can cause serious damage to the neck and body.
I have been rear ended 3 times in my life time... once in my corvette that shattered the rear fibreglass panel and bent the frame... the lady that hit me didn't even try to apply brakes and knocked me across an intersection.... after the car was fixed a guy in a Cadillac smacked the bumper again, but no damage and a lady in an SUV smashed into the rear of my VW and smashed up the deck lid.... fortunately only injury was in the corvette and my head hit the top of the hard top and left a goose egg... none of the others were injurious.