Cheap, Easy Way to Step Down 12-14VDC to 5VDC?

Google buck switching regulators if your intent is to power the microcontroller. There are integrated circuits available that require minimal external components. And of course you can purchase them ready-made too.

If you want to use the existing bulbs, and they are ground-switched, a simple resistor and zener diode at each lamp can lower the voltage to acceptable input levels for your microcontroller. Or, you can use a transistor with a base resistor connected to the ground-switched lamp, emitter to circuit common, collector to a +5 V logic supply rail through a 1 to 10 KΩ pull-up resistor. This would be my preferred implementation. If you use an NPN small-signal transistor, it will be on and saturated when the lamp is NOT illuminated because the transistor base will be forward biased through the non-illuminated light bulb with a few microamperes of current. Connecting the ground-switched side of the lamp through a switch or sensor to ground will illuminate the lamp and turn the transistor off, causing the pull-up resistor to present a logic "1" level at the collector. You connect the collector as a digital input to your microcontroller.

It really isn't necessary, but you could also place an optical isolator's input LED in series with the transistor collector, lowering the resistor value sufficiently to turn the optocoupler's output transistor on when the transistor connected to the lamp circuit is conducting. The optocoupler transistor then replaces the transistor connected to the lamp circuit.

You may not need a buck switching regulator to power the microcontroller, but if you plan on using a lot of LEDs or other displays, along with data logging, the current requirements will blossom upwards and a series regulator or zener diode with current-limiting resistor to drop the motorcycle supply voltage down from 12 to 14 V DC to 5 V DC (or less for some micros) will be inefficient and waste power you need to dissipate as heat.