Simply having a ground wire on your vehicle when it's not in use isn't enough, by itself, to protect your vehicle's electronics from frying in an EMP/EMB wave.
These EMP/EMB waves are likened to being struck by lightening because they carry much the same power in a type of massive static charge. But only using part of the spectrum of energy that is actually found in lightening, by using only some of the frequencies found in lightening, the amount of energy that's required to create it is less, this also allows these wave weapons to be better controled or directed.
In other words; a wave weapon can still be used to over-power your vehicle's electronic systems even though it's very well grounded, causing many vital components to melt-down. The ground on the vehicle simply allows the wave's charge to continue to the earth where it disappates... after doing it's intended damage to the vehicle.
By adding a Farayday Cage sys to the vital electronics of your vehicle will guard from the wave's damage and the secondary protection of grounding the vehicle will reduce any remenant static charge from doing any damage. By 'closing the circuit' in the wave path and by-passing the vital electronics of your vehicle, so to speak.

Using a piece of chain as a ground is better than nothing, but any chain may only work as well as each and every link properly connects with the other in the chain. Then there's the actual contact with the earth, itself, this connection may only be intermittent, at best,(especially if the vehicle is not stationary at the time-duration of the wave attack).
If a 'ground wire' is going to be used to drag from a moving vehicle, I would recommend using somthing like a 3/16",(or slightly bigger), steel cable that's bolted to the chassis and more consistantly contacts the earth as it drags from the vehicle. Even then, this system is not an ideal ground, but it is better than chain, and nothing. IMHO

IHTH

Michael

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P.S. On those TA/312 phones, every micro-inch of the phone's line and each phone unit themselves need to be Faraday Cage guarded or the EMP/EMB wave will find/use any unguarded wiring as an antenna to fry any electronic circuitry in the phone's system. One way to guard those types of phones would be to use a double or triple insulated wire that travels to a breaker system as it passes into the phone's protective faraday cage unit(s).
I still tend to think that using a good short-wave radio system that is well/completely guarded by the faraday cage system and antenna fuses/breaker system is more workable and maybe easier/cheaper to setup and maintain. Again, IMHO.

A sure-fire way to test any electronic system for EMP/EMB waves would be to similate a 'Lightening Strike' on the electronic units/systems. Testing in this way is a 'distructive test', but by doing some kind of test,(even something 'non-distructive'), prior to an actual attack gives you the chance to replace, rebuild, refine the guard of your electronic systems, before you have no other choices due to some wave attack.

IHTH