To re-iterate, the original problem was RFI getting into the vehicle electronics.
As far as ground loops go, we are talking DC ground... the best method is to eliminate RF from the picture by using a dummy load instead of an antenna. With this you will still have the same amperage draw from the vehicle, but no radiated RF. If you still have trouble, then its certainly a DC power problem, such as ground loop. But if you only have trouble when transmitting into the antenna, then its purely an RFI problem and not a ground loop. But the only way to be sure is with a non-radiating dummy load.
I think if it is a ground loop problem, then life would be easier.
Obviously you know the first item about RFI is bonding the vehicle chassis with braided straps.
These straps need to be mounted on sanded surfaces, so they're attached to bare metal instead of paint or oxidization. The first place to strap would across the engine hood hinges, then the engine itself, then the exhaust tailpipe. The tailpipe is mounted on plastic & rubber so vibration doesnt cause rattling.
Snap-on ferrites do help, but it often requires many of them to be effective, since they arent a single piece, their effectivness is reduced. It would be helpful to know the signalling method used by your computer, because it should be CAN bus which is differential and that means common mode interference should be reduced. Maybe the vehicle is too early for CAN bus.
399: correct, the 2950/2970 has a separate chassis and RF ground. I think thats possible because it only operates on 12/11/10 meters. HF rigs cant be that way since they operate on much lower frequencies.
