Buck Regulator Supply From Multiple Sources
As Dave Tweed pointed out, the logic will be wrong the way you drew it. Suggest adding a TVS and fuse on the input to protect the regulator and wiring. Use 200V or higher ordinary diodes (eg. 1N4004) for your input 'or' gate. Consider adding a small series fusible resistor before the TVS to help it clamp. This is 'low side' switching so you need to provide a return wire for the lights rather than the chassis. To make high side switches requires another transistor for each LED bank. 

You do need to protect the MOSFET gates and probably to provide some kind of pull-down. Something like this: 







simulate this circuit &ndash; Schematic created using CircuitLab

Resistors can be 1/4-W 5%. 

If the lights are also connected to a computer or something to indicate bulb failure you may have strange things happening (this stuff started in the 1980s so the car could still be plenty old).