State of Montana provides a credit of 50% of the cost of PHEV Conversion

Alternative Fuel Vehicle (AFV) Conversion Tax Credit Businesses or individuals are eligible for an income tax credit for up to 50% of the equipment and labor costs for converting vehicles to operate using alternative fuels. Qualified alternative fuels are compressed and liquefied natural gas, liquefied petroleum gas (propane), hydrogen, electricity, and fuel containing at least 85% ethanol or methanol. The maximum credit is $500 for the conversion of vehicles with a gross vehicle weight rating (GVWR) of 10,000 pounds (lbs.) or less, and $1,000 for vehicles with a GVWR of more than 10,000 lbs. The credit is only available during the year that the business or entity converts the vehicle. An alternative fuel seller may not receive a credit for converting its own vehicles to operate on the alternative fuel they sell. (Reference Montana Code Annotated 15-30-164)