Well for me, I also have a older car — 2001 model — and now have just mounted my new iPad! (woohoo!) I have only two cables, and they're neatly tucked away.
The biggest issue around this whole concept isn't the mounting or the wires or the cost of the Navigon app.
Instead, it's the MONTHLY DATA PLAN costs. I'm cheap. I don't own any other mobile device except for the iPad3. So I have to manage it. I get Verizon's $20=1GB for one month, which is twice as much as I need (for the week that I go mobile about three times a year). No way am I gonna pay 12 months for any data plan!
So the iPad solution is way way more expensive compared to an old hand held Garmin GPS that only consume AA batteries. This is a very interesting discussion.