This invention relates to a parallel hybrid electric conversion kit for a vehicle with an internal combustion engine, a driveshaft and a transmission, such as, rear wheel drive vehicles, four-wheel drive vehicles, heavy duty multiple-driven-axle vehicles and/or all-wheel drive vehicles. The conversion kit includes a motor-generator, a torque coupler, a battery, a power electronics module and a controller. Suitable torque couplers include transfer cases and/or rear-through differentials. The conversion kit provides an aftermarket solution to increased performance, fuel economy and/or reduced emissions by modifying the drivetrain after the transmission. According to one embodiment of this invention, the control scheme of the installed conversion kit receives input signals from the engine and the motor-generator, but only sends control signals to the motor-generator, facilitating installation of the conversion kit.