I don't know how the add-on units work, but you need one that gives you audio instructions.
The beauty of the in-dash units in cars is that while they show you a map and gives you directions on the face of the radio, the verbal instructions are the only useful part.
As you're driving, a voice tells you what to do while you're driving. It's amazingly accurate. It doesn't make mistakes. If you miss a turn, it will recalculate directions to get you on your way. You end up never looking at the head unit because you are driving and don't want / need to take your eyes off the road.
I don't know if the portable units work this way, i.e. integrate into your car's speakers or have a built-in speaker, but if they don't - I wouldn't buy it.
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Paul S.
2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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