I figured out the theme fairly quickly. Each answer contains a play on those words.
19A. Entrees for one of Dion’s backup singers? = Belmont steaks
30A. Discounts at garages? = Parking brakes
36A. Severely reduced wagers? = Chopped stakes
52A. What mechanics may do as part of a tuneup? = Service brakes
19A. first tipped me off to the theme. Dion and the Belmonts was one of my favorite rock ‘n’ roll groups in the 1950’s.
Completing an error-free puzzle came down to filling in one letter. The problem was that I didn’t know the correct answers to the two intersecting clues containing that letter. So, I left that space blank from Thursday until this morning when I decided to just guess. And I guessed right!
17A. Wing part = AILERON
5D. Rookie: Var. = TIRO