Thank you!  Sorry it took me so long to get to your question since it's a good one.   Car window glass does not block all UV rays as you pointed out.  Window glass usually blocks UVB but not UVA.  So..... using sunscreens that block UVA well becomes even more important if you are in a car a lot.  Make sure your sunscreen has a very high percentage of zinc like the Dermaquest Zinclear(18%).  Using a mineal powder makeup over that like the Jane Iredale (SPF 20) wouldn't hurt.

As for your question on tretinoin in your particular situation, there is really no data on that but as long as your suncreen blocks UVA well and you reapply, you should be fine using it.  And it's fine to use tretinoin in the summer, if you're not outdoors a lot.  I have 3 teenage children and I like to swim in a nearby lake so I use the tretinoin a lot less in the summer.   Dr. I