Where they found a battery with enough juice to power a GPS (Radio) device for the months required to cross the ocean
Well, that took 5 seconds. The third result of a Google search for "long life GPS cellular tracking (http://digitalmatter.com/Devices/Remora) is a non-descript device which features a 5 year battery life with once-per-day tracking, which seems more than adequate for this. If you don't like that one, the results of that search are filled with others.
through the hull of a ship
The location while in transit across the ocean isn't relevant to this study, so the device doesn't have to transmit through the hull of the ship. It just has to be able to continue transmitting once it's been offloaded at the destination so that the destination can be identified.
then have the GPS unit pass undetected through customs
We're talking about containers stuffed with used and broken electronics being delivered to countries that accept them as garbage to be recycled, so that raises three questions:
1) What the hell makes you think the destination countries have customs agents thoroughly inspecting tons upon tons of incoming garbage?
2) How would these hypothetical garbage-inspecting customs agents identify one particular electronic device buried in a pile of other electronic devices as something unusual?
3) Finally, assuming that there is some government that's willing to accept electronics waste (and all its hazardous components) for recycling, yet somehow still anal-retentive enough to inspect said electronics and wealthy enough to be able to pay for the metric ass load of customs agents it would take to inspect and identify everything in the shipment and determine that the GPS tracking was not broken and was active and somehow magically determine that it's being listened to, why would they care? Tracking shipments is a normal thing for that shipping companies do on a regular basis.