If the cargo box is covering the spoiler over the rear window, it will interfer or completely block the reception. When the car is parked indoor, look at the right upper corner of the GPS screen, you can see a globe picture but no green dots to the left of the globe, the car will retain its last position before losing reception when indoor and not moving. Imagine if you go through a tunnel, say in Boston area... you will loos your guidance, and that is when the GPS goes to trip route (if you have a destination set), it will show a text version of the guidance list (like L on abc street in 2 miles, R on xyz Blvd in 1.5 miles and so on) until you are out of the tunnel, the GPS will regain access to the sky and the map reapears. DO not obstruct the GPS antenna view to open sky or you will get reduced accuracy from 20 to over 600 feet or you might completley loos the map display until the antenna gets a better reception. The spoiler itself is NOT the antenna but the antenna is either on the R or L side of the spoiler based on the L or R driver side (like in UK). It looks round omni-directional antenna. By the way the info above apply to MK4 screen and High version of map dvd.