dcdd1 - as you have suggested - you have a 'earth' or ground problem.
You should check (remove, inspect, clean); any ground straps on your vehicle. This actually probably won't fix the problem. But, at least your checking the grounds.
You might also have a bad Radio Frequency Resistor. Which is used to help keep out high frequency energy noise from the radio.
Vehicles generate a tremendous amount of high frequency noise. This is caused by the NUMEROUS sparks or arcs of the spark plugs. We also now have a vast array of induction coils, and IC circuits - to run various computer controlled engine components.
Did you purchase this vehicle new? Are you familiar with the maintenance history, etc? A bad ground can cause an increase in the generated high frequency noise; but if checking the grounds doesn't help; you should probably take it to a shop.
Also - what is the age of your GPS?? Is this new? It is unlikely; but also possible - that the shielding in the GPS is faulty. (They do have shielding.) So, if this is a new unit; you might look at possibly trying a replacement.
I only wonder about this - since the GPS is experiencing interference even when not connected to the vehicles powers source. Usually with a bad ground or something; this causes noise in the 12V line; which then interferes with a device.
However, your device is experiencing interference just from proximity to your vehicle. And, finally, although the device does have some shielded components; the FCC requires that all of these types of devices 'accept' harmful interference.
From your use of the term 'earth' - I suspect your not residing in the States; but still the basic design of the unit probably conforms to the FCC requirement. However, even with all of that - it is either a defective GPS; or some defect in your vehicle. And if the GPS has defective shielding - your vehicle could easily interfere with it - especially if your vehicle has some difficulties - since cars generate a TON of high frequency noise.
Hope this helps.
D.