I need some suggestions please!!
Yes, it's actually quite easy.. depending on how far away your subject is. You just need to solve one problem at a time. Here's how I'd do it.....
First sort out the background. Set the camera on manual and, say, f8 then find the correct shutter speed to get the effect you like that balances the car light blur and the ambient light. Let's say it comes out at 2 seconds.
Now you just need to light your subject with a flash (preferably off-camera), adjusting the output until it's exposing correctly at f8 (or whatever aperture you're using). As the previous poster said, adjusting your shutter speed will change the effect of the car lights and the ambient light, but won't change the effect of the flash. That's what let's you control subject and background individually.
So first decide an aperture and stick with it, then adjust the shutter speed for the background and stick with it, and finally the flash power for the subject. One thing at a time. Then shoot.
If the subjects a long way away then you'll need to trigger the flash remotely. If you don't have a trigger you can ask a friend to hide out of shot and fire the flash when you shout or signal somehow. If your exposure is long enough you'll have plenty of time to do this.