Back then all police cars were "black".

Look inside does definitely mean to look under the hood. You didn't look under the hood at the auction. You bought em' cheap and "asis". Looking under the hood meant you didn't know what you were doin at an auction. Everybody knew who the shiners were and veteran shiner would nevr look under the hood. Cops would note who would look under the hood as a newbie and that's how first arrests were made.

As for the line "shot a coat of primer" - Shiners used primer for one reason. It's not shiny. Black primer doesn't reflect light and when you're getin' run down by the sheriff you get away as fast as you can, raise as much dust as you can skid out off a side road, ditch yer lights and let the sheriff roll right by you. If the sheriff realizes he's passed you up he'll come back shining his searchlight both ways looking for the first shiny reflection to nail you.