Hahaha, actually I'd be a little put off if the driver had his navigation all set and it was like "turn left here", if you know what I mean. I've seen a driver check his navigation one time (that I can remember)... so I think you're totally right about how it's just like a map to look at.
I think I'm a pretty annoying person when I'm in a good mood, so sometimes I've tried striking up conversations with some drivers and they're STILL not talkative. It's like trying to get information out of a spy or something haha.
"Do you have any favorite restaurants in the area?"
*Textbook Response* "They're all quite good"
"Come on you've gotta have a favorite restaurant!"
"Hmm...."
And then I've had a couple of times where the person will give all kinds of information on the area... more than I ever wanted to know. I learned the "清水のぶたいから。。。” phrase from a taxi driver! To be honest though, most of the time they're pretty dead quiet. While normally I'd just attribute it to me being a foreigner or just a weird person or something, most of the time people go out of their way to talk to me (which my closer friends have attributed to my たれ目, which is apparently very approachable to Japanese people). Who knows though
. I guess it's just the luck of the draw
.
I know you said you've travelled all over Japan so it's not really a regional thing, but I'm positive I've ridden in a taxi whose driver had never driven a foreigner before, so there could be that aspect of it. I've found that the drivers in Kyoto or in the Kansai region are a little friendlier while my escapades in Nagoya or my home-turf haven't been as fun. (no offense intended to you and your location though).