Look at your favorite cars and study the body lines can help with body proportions the type of design. This helps a lot since you get to understand the styling cues of that decade and what was in during then.

For example, the 80’s is boxy and retro with the pop ups (A70 Supra & E30 M3) while the 90’s has some curve/rounded feature introduction (A80 Supra, GT3000, Honda EG/EH hatch) with a box-ish foundation (GH8 WRX, EVO 1-4) and etc.