If the cables are short then RG-174 is fine. For longer cables (like reaching inside the car) then I'd use
RG-58 for lower loss. (Doppler systems tend to be lacking in sensitivity to start with.)
You might get some ideas from K0OV's improved doppler antenna / switching system here:
http://www.homingin.com/newdopant.htmlCertainly all the coax lengths have to be identical for the system to work properly.
Oh, and don't use a foam-dielectric cable such as RG-8X if you are going to be running it into
the car through the door jam - the foam is much easier to compress when slammed in the door
than the solid dielectric in RG-58.