Red = hot sounds good. black = ground, maybe not, The white maybe ground, the black might have to connect to the head lights hot wire between the on/off switch and the dimmer switch. This way when the headlight is off, you get a bright back light for daytime running. When you turn on the headlight, the black wire sences headlight power and the display dims for night time.
A lot of car radios are set up this way. I had an old Supra that the clock worked that way. I figured this out when I swapped in a digital clock for the analog one. Decided I liked the anolog better and swapped back.
On your 78 does it still have the headlight switch? And does it turn the headlight of/on? If so wiring it the way I described should work ok.
Leo