While several automotive Original Equipment Manufacturers (OEMs) develop vibration sources such as electric motors internally, many outsource the development and manufacturing of motors to suppliers. In both scenarios, having clear and well-defined vibration requirements for motors is critical during the development phase. This not only sets clear goals for motor development but also clarifies the division of responsibilities between the supplier and OEM, or among different development departments. However, determining the exact values for these vibration requirements is difficult and has not been addressed extensively. This paper tries to improve the accuracy of defining these requirements by combining a) advanced vibration modeling and b) numerical methods for calculating target values. For vibration modeling, substructuring is used for coupling all structures in the vibration transfer path. The source excitation is modeled with component-based TPA, where the source excitation is represented by blocked forces, which are a property of the source component alone. This enables a decoupling between the source component (e.g. the motor) and the transfer path (e.g. the vehicle). The connection between the source component and the rest of the vehicle often has many degrees of freedom (DoFs), making it hard to analytically derive target values for source excitation. To address this issue, the paper employs numerical calculations to determine a solution space for each variable. Finally, the paper presents the proposed method through analytical examples. This research is currently in its early stages. Future work will focus on applying the method to industrial data of electric motors and steering gears. This research aims to improve the accuracy in defining vibration requirements. This is vital for the prevention of late phase trouble shooting and expensive redesigns.