I'd say just run one wire and ground to the car body.
Look at how many thousands of emergency vehicles out there ground to the body.
About 97% of all the Motorola trunk mount radios are grounded at the body.
My personal vehicle is setup with three 110 Watt Motorola Syntor X9000's. these are trunk mount radios. They are all fed of one large B+ lead direct from the battery. Have ran this setup for years, no problem.
granted, I dont run HF in the mobile, but I'd try the single wire, if it works...go with it.