The 90+ radios were designed so that when stopping at a red light & shuting the engine off (mandatory in some euro countries) you could still hear the radio for approx. 5 sec until the key was recycled. There isn't any wiring change as the radio was modified.
The 1988 & 89 radios used a diode that held enough power to keep the memory; just like a computer when it's unplugged.