can i hot wire battery operated ambulance and police cars etc, to 9 volt dc power or 4.5 dc
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Not exactly clear what you are trying to achieve? Can you provide a little more info. The cars descriptions current power requirements etc.
Yeah, this is a 3 y/o Menards vehicle at 4.5vdc and a relay. Took it apart and hard wired. Just total the voltage of the batteries and you should be good.
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Occasionally the toy won't like being fed from anything but a battery or SUPER clean power source, but most work on a wall wart or on track with a bridge rectifier or diode. Things with sound triggers seem to have the most issues.
E.g. My enigines passing close will trigger some.
Capacitors act to smooth an ac to dc conversation by BR or diode, closer to the constant output of a battery.
Mind voltage, leds especially don't like over voltage. Knocking voltage down, add a gen. purp. rect. diode inline(series), they eat .5 to.75v each.
I have this same Menard's ambulance. I found that the flashing lights lasted only a short time with the included button cell batteries. I replaced them with 3 AAA batteries in an old flashlight battery holder and the lights run forever. Have yet to replace the batteries.
Three AAA or AA in series might give you 5v when new (1.7v-1.8v each simetimes) Maybe a bit too risky for for 4.5v volt items. I'd add a diode or run cheap dry cell batteries 
(no wet chemical leaks either )
When you aren't sure what voltage is safe, start low and stop once things work, 1.5v, 3v, 4.5, 6,9,12v will be the normal limits for the most common leds. Esp. common is 1.5v-4.5v.
And don't assume voltage, measure it.
