The simple way is to add some cross-channel antiphase to both channels. So R = (R - cL), and L = (L - cR) where c << 1. This is very cheap and easy with opamps

The simple way is to add some cross-channel antiphase to both channels. So R = (R - cL), and L = (L - cR) where c << 1. This is very cheap and easy with opamps. It does indeed sound magical, and makes the stereo image expand beyond the speakers. It also does weird things to a mono mixdown and if c is too big you get a hole in the middle. But if you keep c small and use it as the final effect just before the speakers that's not a problem. You could also use BBD[1] chips to add a ms or so of analog delay, but that's less likely because it would have been more complex and expensive. Digital audio delays had appeared in studios by the late 1970s, but they were still more expensive than BBDs in the mid-80s, so unlikely for in-car use. [1] Bucket Brigade Delay