Reducing PWM voltage peak
Probably doable very simply.
Definitely doable with not TOO much effort.  

2 resistors should work OK.
For 12v-5V with no loading by the output, the resistors would be in the ratio 7:5 (7V dropped across the top one, 5V across the bottom for 12V total).
7/5 = 1.4:1
So eg a 1500 Ohm and 1000 ohm (12 to 4V)
or 1500 + 1200 (12 to 5.333v).
Depending on how variable the 12V is and how critical the 5V is such a simple solution may work. 

If the output is low resistance it may load Vout down. This can be compensated for by changing resistors. A zener diode "clamp" on the output keeps the Vout more constant and provides a degree of protection against over-voltage.   







simulate this circuit &ndash; Schematic created using CircuitLab

If the 12V may be eg 11-14V and/or the %v must be 5 +/- 0.1V or ... then more precise solutions are not much more complex. If you have a better guide to voltage ranges and if you can provide vehicle and equipment model/brand details it may help.

Safer methods which better isolate the 12V from the 5v circuits can be designed if desired.