As an electrical engineer in the automotive industry, I can tell you that the best thing to do would be to run the ground up to the engine block. Running it through the frame has more resistance and therefore more voltage drop. You also lose power on power circuits. The frame still needs to have a connection to B- though because other circuits in the car are grounded to the frame.
As a managing electrical engineer formerly in the automotive industry, as an aftermarket ancillary electronics equipment designer, and as someone who routinely solves problems other people cannot seem to fix, I disagree.
The body shell in a typical unibody car has FAR less resistance from from to rear of the vehicle than any practical cable could ever have. I have seen this over and over again with direct measurement. It also calculates that way if you compare resistivity of steels to copper, and then calculate the cross sectional area of the body and compare it to a practical cable.
Not only does the body have less resistance, it has radically lower impedance for high frequency signals like noise.
It is foolish to run a cable from a trunk battery to the block. I can *prove* this with either measurements or by calculation, as can anyone who know how to measure things.
The best possible noise ground and vehicle distribution in a unibody car is the chassis itself. If the battery is near the engine, then a block connection makes sense for the start and charge currents ONLY. Any distribution should be chassis centric, and any sensitive devices should be chassis centric.
To my point is this:
I'm 99% sure I have found my issue. The ECM ground wire behind the passenger side kick panel looked fine, but I had never removed it before. Today when I took it loose it was a nice shade of green. Cleaned it up and now the car runs consistent enough to try and dial everything in. Good grief I know better to overlook ground wires and this one cost me a week's worth of head scratching, but oh what a relief.
Ford uses the CHASSIS as the noise reference ground. They do this for good reason. If you lose that chassis connection, even though the EEC is grounded to the fender at the battery ground point, the system will generally noise up.
If the battery is in the rear, the block negative return becomes too long. The best solution is a good solid short block to subframe ground in front, and a good solid ground from the battery negative to frame or shell at the battery.
Nothing should ever tie to a battery negative except the block ground and/or the chassis ground. Never. The only thing going to the battery negative should be the block and/or chassis ground.
My 1989 LX coupe has a 00 cable from battery to front. That 00 cable has about three times the resistance of my chassis path. This is using a four wire measurement technique. My car is chassis centric grounded.
If my battery was in front, the starter and charging would be block centric and the chassis would be the distribution for all other electronics.