Hello Paula I can help with your project, and others may reply also. Please post some images of the model.
You say you have created a quarter car model in MATLAB, which implies just one wheel. I would like to know what you learned from that.
You have also looked at a Modal analysis and you say you fixed the rear wheels. What did you do with the front wheels? Please show the constraints you used in your Modal analysis. What was the total mass in the model? How many springs did you include in the model? What was the spring rate? Did you use any Joints in the model> Please show them.
Let's just focus on creating a valid Modal analysis before we worry about the base excitation.
Regards Peter